Life On Other Worlds

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LIFE ON OTHER WORLDS

WORLDS WITHOUT END

BY
THE ASTRONOMER ROYAL

" His charmingly lucid book."

H. O. WELLS

"A coherent survey, concisely and
precisely made, of all that any intelli-
gent reader should wish to know."

SIR RICHARD GREGORY

" Written with singular clearness, it
gives a most admirably balanced
popular survey of current astronom-
ical knowledge and speculation."

THE BISHOP OF BIRMINGHAM

Published by THE ENGLISH UNIVERSITIES

PRESS at 55. net, and in the HODDER

AND STOUGHTON Black Jacket series at

2J. fid. net

LIFE ON OTHER WORLDS

BY

H. SPENCERJQNES..

M.A., Sc.D., F.R.S.

ASTRONOMER ROYAL
HONORARY FELLOW, JESUS COLLEGE, CAMBRIDGE

THE ENGLISH UNIVERSITIES PRESS LTD.

LONDON

FIRST PRINTED . . July 194
REPRINTED . . November 194

Printed in Great Britain by
Hazell, Watson & Viney, Ltd., London and Aylesbury.

PREFACE

IN his Preface to La Pluralite des Mondes, Bernard
de Fontenelle (1657-1757) wrote:

" I have chosen that part of Philosophy which is
most like to excite curiosity; for what can more
concern us, than to know how this world which we
inhabit, is made; and whether there be any other
worlds like it, which are also inhabited as this is ?
They who have any thoughts to lose, may throw
them away upon such subjects as this; but I suppose
they who can spend their time better, will not be at
so vain and fruitless an expence." (John GlanvilPs
translation of 1688.)

The question whether life exists on other worlds is
Che that still excites curiosity, and to which astrono-
mers are expected to give an answer. In this book I
have summarised the evidence, provided by present-
day astronomy, which has a bearing on this ques-
tion, and have attempted to give an answer. I do
not expect that every reader will agree with the
conclusions that I have reached; it is open to each
reader to form his own conclusions, provided that
they do not conflict with the evidence derived from
the astronomical observations.

The most serious- difficulty in discussing whether
there is life on other worlds is, naturally, our
ignorance of how life originated on the Earth. I
have assumed that wherever in the Universe con-
ditions are suitable for life to exist, life will somehow
come into existence. The fact that there is pretty

VI PREFACE

conclusive evidence of the presence of vegetation on
Mars, the only world where from a priori considera-
tions we are able to conclude that conditions are
suitable for the existence of life, lends some support
to this assumption. There are those who will not
consider this assumption to be justifiable and others
who may regard it as opposed to religion. For the
latter, I will quote again from de Fontenelle :

" There remains no more to be said . . . but to a
sort of People who perhaps will not be easily
satisfy'd; not but that I have good reasons to give
5 em, but because the best that can be given, will
not content 'em; they are those scrupulous Persons,
who imagine, that the placing Inhabitants any
where, but upon the Earth, will prove dangerous to
Religion : I know how excessively tender some are in
Religious Matters, and therefore I am very un-
willing to give any offence in what I publish to
People, whose opinion is contrary to what I main-
tain. But Religion can receive no prejudice by
my System, which fills our infinity of worlds with
Inhabitants, if a little errour of the Imagination be
but rectify'd. . . . And to think there may be more
Worlds than one, is neither against Reason, or
Scripture. If God glorify'd himself in making one
World, the more Worlds he made, the greater must
be his Glory. 55

The first chapter presents the picture of the
Universe provided by modern astronomy, serving
as a background against which the problem under
discussion may be viewed in its proper perspective.
With the Universe constructed on so vast a scale,
it would seem inherently improbable that our

PREFACE Vll

small Earth can be the only home of life. In the
second chapter the conditions for the existence of
life are considered. These conditions are based
on the essential uniformity of matter throughout the
Universe. The same atoms, subject to the same
chemical laws, are found in the Earth and in the
remotest star or nebula. In living matter, the
chemistry of the carbon atom, intimately bound up
with its peculiar property of forming in combina-
tion with other atoms a multitude of large and
complex molecular aggregations, is of particular
significance. The simplest living cells are highly
complex, and correspondingly fragile, organisations.
The conditions for such complex molecular aggre-
gations to be possible are very restricted. The
question whether or not the various planets in
the solar system comply with these conditions is
discussed in detail in later chapters.

But before embarking on this discussion, an out-
line of the methods of investigation that are open
to the astronomer is given in Chapter III. From
purely theoretical considerations it is possible to
infer whether a planet is likely to have an atmo-
sphere or not and also to estimate its temperature
within fairly narrow limits. These conclusions can
be checked by observations. Information about the
composition of the atmosphere of a planet can be
obtained both from general considerations and from
direct observations.

The atmosphere of the Earth and its evolution is
discussed in Chapter IV, in relation to the con-
clusions reached in the preceding chapter. The
presence of free oxygen in the atmosphere of the

Vlll PREFACE

Earth, of particular importance for the existence of
animal life, is attributed to the widespread vegeta-
tion over the surface of the Earth.

The detailed discussion of the different members
of the solar system shows a wide diversity of condi-
tions; for the most part these conditions are such
that the possibility of the existence of life can be
definitely excluded. Venus appears, however, to be
a world where life may be on the verge of coming
into existence and Mars a world where life is in the
sere and yellow leaf.

Passing beyond the solar system, the question
arises whether other stars can be expected normally
to have systems of planets associated with them.
In order to answer this question, the origin of the
solar system must be explained. This has proved
to be a difficult matter. The various theories are
discussed in Chapter IX, and it is concluded that, if
the distribution of the stars at the time the solar
system was formed was very much as it is at the
present time, such a special combination of circum-
stances is needed to account for the solar system
that it can be inferred that relatively few of the stars
have families of planets.

In the final chapter these conclusions are dis-
cussed in relation to the picture of the Universe
given in the first chapter. Though conditions are
heavily weighted against suitability for the exist-
ence of life, it is concluded that there must never-
theless be many other worlds where the appropriate
conditions are to be found and where therefore we
may suppose that life in some form or other does
actually exist.

PREFACE IX

I am greatly indebted to Dr. W. S. Adams,
Director of the Mount Wilson Observatory, Cali-
fornia; to Dr. W. H. Wright, Director of the Lick
Observatory, Mount Hamilton, California; and to
Dr. V. M. Slipher, Director of the Lowell Observa-
tory, Flagstaff, Arizona, for permission to reproduce
photographs taken at these observatories and for
kindly supplying photographs for this purpose. I
am also indebted to the Rev. T. E. R. Phillips for
permission to reproduce two of his excellent series of
drawings of the planet Jupiter.

H. SPENCER JONES.

CONTENTS

CHAP. PAGE

PREFACE ...... V

I. A PICTURE OF THE UNIVERSE . . I

II. THE CONDITIONS FOR THE EXISTENCE OF

LIFE . . . . . .21

HI. METHODS OF INVESTIGATION . . 53

IV. THE EVOLUTION OF THE ATMOSPHERE OF

THE EARTH ..... 78

V. WORLDS WITHOUT ATMOSPHERES . IO2
VI. THE GIANT PLANETS . . . .122

VH. VENUS THE EARTH'S TWIN SISTER . 152

Vin. MARS THE PLANET OF SPENT LIFE . 172

IX. THE ORIGIN OF THE SOLAR SYSTEM . 212

X. BEYOND THE SOLAR SYSTEM . . 244

LIST OF PLATES

PLATE FACING PAGE

1. STAR CLOUD IN THE CONSTELLATION OF

SAGITTARIUS (THE ARCHER) . . xlv

2. THE GREAT NEBULA IN ORION . . 1 6

3. THE MILKY WAY NEAR THE STAR RHO

OPHIUCHI . . . . .17

4. THE MOON! THE APENNINES AND ARCHI-

MEDES ...... 32

5. THE MOON! REGION OF TYCHO AND

CLAVIUS ...... 33

6. THE MOON: REGION OF COPERNICUS . 80

7. THE PLANET JUPITER . . 8 1

8. THE PLANET SATURN .... 96

9. THE SPECTRA OF THE MAJOR PLANETS . 97

10. SPECTRA OF THE PLANETS . . .144

11. CLOUDS ON THE PLANET VENUS . .145

12. PHOTOGRAPHS OF MARS AND OF TER-

RESTRIAL LANDSCAPE . . l6o

13. CLOUDS ON THE PLANET MARS . . l6l

14. THE PLANET MARS .... 2O8

15. THE SPIRAL NEBULA 5 MESSIER IOI, IN THE

CONSTELLATION OF THE GREAT BEAR . 2OQ

1 6. SPIRAL NEBULA IN THE CONSTELLATION OF

BERENICE'S HAIR . . . .224

17. SPIRAL NEBULA, MESSIER 8 1, IN THE CON-

STELLATION OF THE GREAT BEAR . 225

PLATE i

STAR CLOUD IN THE CONSTELLATION OF
SAGITTARIUS (THE ARCHER)

The photograph shows a part of the brightest region
of the Milky Way. The denser parts of the cloud are
formed by a continuous background of faint stars,
which are so numerous that their images blend together.
There are numerous dark lanes and channels visible
over the cloud, caused by patches of obscuring dust.

The centre of the Milky Way is in the direction of
Sagittarius, so that we are here looking through our
stellar universe to its greatest depths. It is for this
reason that the density of the stars seen in this region
is so great.

The distance of the centre of th$ Milky Way system
from the Sun is about 30,600 l$ght?years, or 180,000
million million miles.

Photograph taken by Mr. Franklin Adams.

CHAPTER I

A PICTURE OF THE
UNIVERSE

THE investigation of the structure of the universe
may be said to commence with the work of William
Herschel (1738-1822). By profession a musician,
Herschel came to England from Hanover at the age
of nineteen to improve his fortunes. He earned his
living by playing the organ and teaching music, and
in 1766 was appointed organist of the Octagon
Chapel, Bath. But all his spare time was occupied
with the study of pptics and astronomy, in which he
became more and more interested. Merely to
learn about astronomy from books was not sufficient
for his eager inquiring mind. He longed to study
the heavens through a telescope with his own eyes.
Being unable to afford to buy a telescope, he
decided to attempt to -make one for himself.

Starting in a modest way, Herschel proceeded to
larger and larger telescopes. It was not long before
he became better at making telescopes than anyone
had been before and, what is more, he showed extra-
ordinary skill in using them. To quote his own
words: " Seeing is in some respects an art, which
must be learnt. 5 ' It was the excellent optical qual-
ity of his telescopes, combined with his keenness of
vision, that enabled him to make the discovery that
brought him fame. On March 1 3th, 1781, he
noticed what he described, in his journal as a
" curious either nebulous star or perhaps a comet, 5 '

2 LIFE ON OTHER WORLDS

an object Showing a small round disk, differing in
appearance from the point-like image of a star.
This object proved to be a new planet, now known
as Uranus ; it was the first planet ever to have been
discovered by man, for the other planets known at
that time are easily visible to the naked eye and had
been known from remote antiquity. The distance
of Uranus from the Sun is twice the distance of
Saturn, the remotest of the planets known before
HerschePs discovery, so that the discovery doubled
the extent of the solar system. It was natural that
it should cause a great deal of interest and excite-
ment; one outcome of it was that the King, George
III, created for Herschel the post of King's Astron-
omer, with a salary of 200 a year. This enabled
him to give up his musical career for good and all in
favour of astronomy.

Free to devote himself entirely to the pursuit of
astronomy and the making of telescopes, Herschel
set before himself the arduous and gigantic task of
making a regular and systematic survey of the
entire heavens. He examined each object that
came into the field of his telescope and noted any
peculiarities about it. He found that the Milky
Way, the belt of diffuse whitish light that encircles
the heavens, was composed of a vast number of faint
stars. By counting the number of stars visible in
the field of his telescope in different parts of the sky,
he gauged the depths of the stellar universe. From
these investigations he came to the conclusion that
the universe was an extensive flattened system,
shaped very much like a grindstone. He believed
that the Sun was somewhere near the centre of this

A PICTURE OF THE UNIVERSE 3

system. The appearance of the Milky Way was the
result of looking through the system, from a central
position, in the directions of its greatest extension.

In the course of his systematic survey, William
Herschel discovered no fewer than 2,500 nebulae and
star clusters. The nebulae were cloud-like objects,
shining with a diffuse light, and he was inclined at
first to believe that they were really composed of a
multitude of stars so distant that his telescope could
not resolve them into separate points of light, just as
the naked eye is unable to resolve the Milky Way
into discrete stars. But gradually he became con-
vinced that many of them were not aggregations of
stars, but masses of glowing gas. This conclusion
was confirmed forty-two years after his death, when
their light was analysed with the spectroscope and
found to show the characteristics of the light from a
glowing gas.

At the same time he did not believe that all the
nebulae were gaseous. He was convinced that some
of them were composed of a multitude of stars and
he regarded these as island universes, comparable in
size with our own stellar universe. He said on one
occasion: " I have looked farther into space than
ever human being did before me. I have observed
stars of which the light, it can be proved, must take
two million years to reach the earth a " This was a
remarkable statement to make at a time when the
distance of not a single star was known.

These investigations and conclusions of William
Herschel have been summarised at some length
because they provided for the first time a picture of
the universe based on a systematic detailed survey

4 LIFE ON OTHER WORLDS

of the sky. Subsequent investigation has confirmed
in its essentials the picture painted by Herschel.
Many details have been added and a few of his con-
clusions have been modified. Not without reason
has he been called the father of modern astronomy
and well merited were the words in his epitaph:
" Coelorum perrupit claustra." He broke through
the barriers of the heavens.

It was not until 1835, thirteen years after Her-
scheFs death, that the distance of a star was
measured for the first time. The principle of the
method is simple enough; it is essentially the same
as that employed by the surveyor to measure dis-
tances on the Earth's surface. The object whose
distance is required is observed from the two ends of
a base-line of known length, the observations giving
the angle between the directions from the object to
the two ends of the base-line. The difficulty of
measuring star distances arises from the fact that
the longest base-line available for the purpose is very
short compared with the distances of the stars. By
making the observations when the Earth is at the
two ends of its orbit, a base-line of about 186,000,000
miles is obtained. No longer base-line is possible.
To appreciate the problem that faces the surveyor
of the skies, let us represent this base-line of
186,000,000 miles by a line two inches long; the
distance of the nearest star on the same scale is then
about four miles. We have to measure, in effect,
the distance of an object four miles away by making
observations from two points only two inches apart!
Still more difficult is it when, from the same two
points, we have to measure distances of ten, a

A PICTURE OF THE UNIVERSE 5

hundred or a thousand miles. Success has been
attained by careful attention to detail and by pre-
cautions to eliminate as far as possible every source
of error.

It is convenient to express star distances in terms
of the time that light takes to travel. Light travels
with a speed of 186,000 miles a second, so that in
the course of a year it will travel a distance of nearly
six million million miles. Thus, for instance, in-
stead of saying that the nearest star is 25 million
million miles away, we may say that it is about
four light-years away. This mode of expressing the
distance has the additional interest of reminding us
that we see the star, not where it is at the moment,
but where it was four years previously.

There is a limit to the distances that can be
determined by direct measurement. For distances
greater than about 500 light-years, the results be-
come rather uncertain. If space is to be explored to
greater distances, it must be by indirect methods.
Such a method, which is of very great power, has
been discovered, and the knowledge that has been
obtained within the last two decades about the
structure of the universe has been acquired in a very
large measure by the application of this method.
It is based on the special properties of a particular
class of stars. These stars do not shine with a steady
constant light; their light waxes and wanes in a per-
fectly regular manner. It has been found that the
fluctuations in the brightness or candle-power of
such a star are accompanied by regular pulsations
of the whole star; the star swells up and contracts
with perfect regularity. The time required for a

O LIFE ON OTHER WORLDS

single pulsation to be completed, though constant
for any one star, ranges for different stars from
several hours to about thirty days ; if the pulsating
stars are arranged in the order of the time of a single
pulsation, it is found that they have also been
arranged in the order of their candle-power. There
is, in fact, a definite relationship between the period
of pulsation and the candle-power of the star, so that,
if the period is known, the candle-power can be
inferred.

There is no great difficulty in finding the time
taken by a pulsating star in going through one
complete cycle of light-variation; we determine this
time, and infer the candle-power, or intrinsic bright-
ness, of the star. We can also measure the apparent
brightness of the star, the brightness as seen by the
eye. The apparent brightness depends on two
quantities the intrinsic brightness and the dis-
tance. If we were to remove a star to twice its
present distance, it would appear only one-quarter
as bright as before. When both the intrinsic and
apparent brightness are known, the distance can
easily be inferred.

The longer the period of pulsation, the greater is
the candle-power of the star. Thus, for instance,
if one pulsation is completed in two days, the candle-
power is 260 times that of the Sun; if completed in
ten days, the candle-power is 1,700 times that of the
Sun; and if completed in thirty-six days, the candle-
power is 9,600 times that of the Sun. It will be
noticed that the candle-power in each of these
examples is very much greater than that of the Sun.
The pulsating stars are all, fortunately, intrinsically

A PICTURE OF THE UNIVERSE 7

very bright; they are included in the class of stars
called giant stars. Their great luminosity makes it
possible to see them far away across the vast reaches
of space, where mediocre stars like the Sun would be
lost to view. It is this fact that renders them so
useful in the exploration of space to great distances.
For example, suppose we discover in a remote star-
cloud that there is one of these pulsating stars whose
pulsation is completed in thirty-six days and sup-
pose that this star appears as a star of the eleventh
magnitude (or about one-hundredth of the bright-
ness of the faintest star visible to the unaided eye).
Such a star would be easily visible in a six-inch
telescope. Because the pulsation takes thirty-six
days, we know that the candle-power of the star is
9,600 times the candle-power of the Sun. Because
it appears as a star of the eleventh magnitude, we
can infer that its distance must be 50,000 light-
years. When we recall that the limiting distance
that can be determined with any approach to
accuracy by direct measurement is about 500
light-years, it will be realised what a powerful
method for the exploration of space is provided by
the pulsating stars.

The general principles that underlie the determi-
nation of great distances having been briefly de-
scribed, the intervening steps can be passed over
and we can proceed to summarise the information
that has been derived about tllfc stellar universe in
which we find ourselves.

The general picture drawn by Herschel, of a vast
flattened system, shaped like a grindstone or a thin
pocket watch, is confirmed in its essentials, but we

8 LIFE ON OTHER WORLDS

now have a far more precise idea of the size and
structure of the system. The plane of the Milky
Way marks the direction of greatest extent of the
system, and it is for this reason, as Herschel realised,
that the stars are most numerous in the Milky Way
regions. The Milky Way is not uniform in bright-
ness, nor is the distribution of the stars in the Milky
Way uniform. They tend to cluster into aggrega-
tions or star-clouds. The brightest region of the
Milky Way, containing the densest aggregation of
stars, is in the constellation of Sagittarius in the
southern sky.

Herschel believed that the Sun was near the
centre of the system. We now know that in this
conclusion he was mistaken and that the Sun occu-
pies a position far out from the centre of the system,
though near its median plane. In other words, the
Sun is a star in one of the Milky Way star-clouds.
The centre of the system, as seen from the Sun, is in
the direction of the Sagittarius star-clouds, and it
is because in this direction we are looking through
the system to its greatest depths that the density of
the stars appears so great. A photograph of a
portion of this region of the Milky Way is repro-
duced in the Frontispiece.

In addition to the stars there are also the nebulae,
rarefied clouds of luminous gas, which are found
only in or near the Milky Way. One of the most
beautiful of these is the Great Nebula in Orion,
shown in Plate 2, which is visible to the naked eye
as a hazy diffuse patch of light. The nebulae are'
not self-luminous; they do not shine by any light of
their own. We see them only by virtue of the stars

A PICTURE OF THE UNIVERSE 9

embedded in them; the atoms in the nebulae absorb
the light from the stars and re-emit it in radiations
of different wave-lengths. Associated with these
bright nebulae there are also to be found dark
nebulae or obscuring clouds. In the midst of some
of the densest star-clouds of the Milky Way there can
be seen blank spaces, completely or almost com-
pletely devoid of stars. A good example of a blank
space is shown in Plate 3. Hundreds of such blank
patches are known. They cannot be vacant lanes
or channels through the stars, for it is beyond reason
to suppose that hundreds of such channels, extend-
ing to immense distances, could point directly to
the Earth. The blank patches are caused by
opaque clouds that lie between us and the stars,
which they obscure from our sight. The opaque-
ness of these clouds is due to the presence of ex-
tremely fine dust. Small dust particles, of a size
comparable with the wave-length of light, have a
very great obscuring power, and if the average
amount of dust in the cloud is only one fifty-
thousandth of an ounce in each square inch of
cross-section, the cloud will be completely
opaque, whatever its thickness may be.

Though the luminous nebulae and the opaque
clouds may each occur separately, it is more usual
to find the two in close association. It seems that
gaseous matter and the larger dust particles are ex-
tensively spread throughout the Milky Way regions;
where the gaseous matter predominates we see
bright nebulosity and, where the dust clouds pre-
dominate, the stars that lie behind are hidden.
Over a great portion of its extent the Milky Way is

IO LIFE ON OTHER WORLDS

divided into two branches; this effect is caused by
the existence in the central regions of the Milky
Way of obscuring clouds of very great extent.
Because the dusty matter is so widespread, we can
never hope to see to the bounds of the Milky Way.
The gaseous matter of the luminous nebulae and
the dusty matter of the opaque nebulae can be
thought of as the residuum of the diffuse gaseous
matter of which our stellar universe formerly con-
sisted. This matter has to a large extent been
condensed into stars ; the stars are continually
gathering in, by their gravitational attraction,
matter from the space surrounding them. They
are gradually sweeping space clean, but the sweep-
ers are few in comparison with the vast regions that
have to be swept, so that the process is yet far from
completion. There are reasons for believing that
the total matter not yet condensed into stars is
about equal in amount to that contained in all
the stars.

The pulsating stars have provided the key by
means of which the dimensions of our stellar
universe have been determined. It is found that
its diameter is about one hundred thousand light-
years and the distance from the Sun to the centre
of the system is about thirty thousand light-years;
the Sun is near the centre of a local clustering of
stars, or a star-cloud. The motions of the stars
have shown that the system is slowly rotating,
under its gravitational attraction. Such a system,
consisting of discrete stars and scattered matter,
does not rotate as a solid body. When a solid body
is in rotation, the motion is more rapid the greater

A PICTURE OF THE UNIVERSE II

the distance from the centre of rotation; a point on
the rim of a rotating wheel, for instance, moves
more quickly than a point on the hub. But in the
heavens the rule is exactly the opposite ; the nearer
to the centre of the rotation, the more rapid is the
motion. This can be illustrated by the planets in
the solar system; the planet nearest the Sun,
Mercury, is moving with a speed of thirty miles a
second; the Earth is moving with a speed of eighteen
miles a second; Neptune is moving with a speed of
only three miles a second.

When the movements of the stars are analysed by
statistical methods, it appears that the stars in a
certain direction have the most rapid motion on
the average, and those in the diametrically opposite
direction have the least rapid motion; it is also
found that the direction in which the stars have the
most rapid motion is the direction to the centre of
the system. These results provide certain evidence
of rotation. But we can learn much more. We
can obtain an estimate of the total amount of
matter in the system, because it is the gravitational
attraction of this matter that controls the rotation.
It is found in this way that the system as a whole
has a mass about 160,000 million times the mass of
the Sun. In this is included the masses of all the
stars, including any stars that may have ceased to
shine as well as the mass of all the diffused matter
scattered throughout the system. It is not possible
to say how many stars there are in the system, but
100,000 million may be taken as a rough estimate,
indicating the great scale on which the system is
built. The time taken by the Sun to make one

12 LIFE ON OTHER WORLDS

complete revolution round the centre is about 225
million years ; the stars in the neighbourhood of the
Sun have .an average speed of about 1 70 miles a
second. Each star has, in addition, its own peculiar
motion relative to the group; the motion of the Sun,
for instance, relative to the surrounding stars is
about thirteen miles a second.

In photographs of the Milky Way star-clouds, the
stars appear to be so closely crowded together that
it would seem that frequent collisions between them
must occur. This appearance is quite deceptive,
however. The stars are so far apart that our stellar
universe as a whole is comparatively empty. We
have seen that the nearest star is 25 million million
miles away, and it is therefore clear that the neigh-
bourhood of the Sun is pretty free from stars. It
might be thought that the Sun is perhaps in a
particularly empty part of the system, but this is not
so. The density of stars around the Sun is fairly
representative of the system as a whole, except per-
haps in the regions close to the centre. Jeans has
calculated that an actual collision between two stars
can occur on the average only once in 600,000
billion years. This is much greater than the age of
the stars, so that we may say that to all intents and
purposes collisions between two stars never occur.

If we were to travel with the speed of light
through our stellar universe, in the direction away
from its centre, we should find after some thousands
of years that the stars were becoming less numerous.
Some time later we should find only a few scattered
outlying members of the system, and at last we
should leave these behind and find ourselves in outer

A PICTURE OF THE UNIVERSE 13

space, free from stars. If we continued to travel
on, should we come to other stellar universes, or is
our Milky Way system the one and only universe ?
We have seen that William Herschel was convinced
that some of the nebulae that he observed were
island universes, at such great distances that his
telescopes could not resolve them into discrete stars.
The analysis with the spectroscope of the light from
such nebulas lends confirmation to HerschePs
views, for the light from these nebulae does not show
the characteristics of the light from a glowing gas,
but is more like the light from the stars.

The nebulae in question are generally called spiral
nebulae, because when seen broadside on they show
a characteristic spiral structure. The typical spiral
nebula has a bright nucleus from which, at diamet-
rically opposite points, two arms emerge and curl
round in the form of a spiral. Such nebulae are to
be found at all angles to the line of sight; some are
seen obliquely, when the spiral structure may per-
haps be traceable, though not so clearly shown as in
those that are seen broadside on. Others are seen
edgewise on, and then the spiral structure is not
evident; but such nebulae appear exactly like what
we have found our own universe to be a flattened
disk-like system. There is an essential continuity
from the nebulae seen broadside-on to those seen
edgewise-on, and we can infer that these latter mutft
also possess the spiral structure. Photographs of
spiral nebulae seen broadside on, inclined to the line
of sight, and edgewise, are reproduced in Plates 15,
1 6 and 17 respectively.

For a hundred yea^s after HerschePs death the

14 LIFE ON OTHER WORLDS

question whether the spiral nebulae were island
universes outside our own universe continued to be
debated. It has been onlpwithin recent years that
the question has been finally settled. The key to
the whole question was to find the distances of these
nebulae, because if their distances were known we
would at once know whether they were inside or
outside our stellar system ; we would also know their
size and would be able to decide whether they were
at all comparable in size with our own system.

The problem was solved when it was found that
within some of these nebulae there were stars which
showed all the characteristics of the pulsating stars.
The nebulae in which these stars were found were
those of largest apparent size and therefore presum-
ably the nearest to us. Their periods of pulsation
were determined and their distances were inferred.
They were found to be of the order of a million light-
years. This was conclusive evidence that the spiral
nebulae were outside our stellar universe and that
they were, in fact, island universes. The close
agreement between the distances of the nearer
external universes and the estimate of distance given
by Herschel (two million light-years) may be noted.

The size of these other universes proves to be of the
same general order as that of our own universe. It
is found also that they are, like our universe, in slow
rotation; they may be thought of as gigantic celestial
Catherine wheels, spinning round, with their vast
spiral arms. They seem also to contain about the
same amount of matter as our own system. The
nearest of the external universes can be studied in
some detail with the aid of the modern powerful

A PICTURE OF THE UNIVERSE 15

telescopes. The typical features of our own system
are shown by them aggregations of stars into star-
clouds, bright gaseous nebulosity and opaque dust
clouds. The obscuring dust clouds are found to be
extensively scattered throughout the central plane
of each system, as they are in the region of the Milky
Way.

These external universes are of all grades of
apparent size from the nearest, which have an
angular diameter of a few degrees, to very remote
systems, whose images on the photographic plate
are scarcely distinguishable from the images of
stars. If we make the assumption that the universes
are all of much about the same size, we can make
rough estimates of their distances. These estimates
have served to establish a remarkable fact, which
can in turn be used to provide a much more certain
determination of the distances. The velocity of
each system directly towards us or directly away
from us can be measured; the principle used for
this purpose is that if a body, which is sending out
radiations in the form of light, is approaching, its
radiations are slightly compressed together so that
their wave-lengths are slightly shorter than they
would be if the body were at rest; if, on the other
hand, it is moving away, the radiation that we
receive are all of slightly longer wave-length.

It is found that the external universes are moving
away from us and that the more distant they are,
the more rapid is their velocity of recession. This
is not the place to discuss the possible interpreta-
tions of this remarkable result. One suggestion is
that the universes are the fragments of one large

PLATE 2

THE GREAT NEBULA IN ORION

This nebula is visible to the naked eye as a hazy
patch in Orion's dagger, below the middle of the three
bright stars that form his belt. The bright portion
measures about six million million miles across, but the
extent of the fainter outer portion is fully three times as
great. The nebula is composed of gaseous matter of
extremely low density, about one thousand million
millionth of that of air under ordinary conditions (much
less than that of the most perfect artificial vacuum), yet
because of its great extent the total amount of matter in
the nebula has a mass about ten thousand times that of
the Sun. The nebula glows by the light of stars em-
bedded within it. The round white dots are images of
stars (other Suns), the large round patch near the top
of the photograph being the image of a bright star.

Photograph by Messrs. Ritchey and Pease, with the
24-inch reflector at the Yerkes Observatory, 1901,
October 19.

PLATE 3

THE MILKY WAY NEAR THE STAR RHO OPHIUCHI

The photograph depicts a portion of the Milky Way
in the neighbourhood of the star Rho Ophiuchi, not far
from the bright naked-eye star Antares in the constella-
tion of the Scorpion. The background of the photo-
graph shows images of a great number of faint stars
faint because of their great distance and not because
they are of intrinsically low luminosity.

In the right-hand portion of the photograph is seen a

sharply defined lane, almost devoid of star images.

This appearance is caused by a cloud of obscuring* dust,

relatively near to the Sun, which hides the distant stars.

The few stars whose images are seen projected on the

cloud lie between the Sun and the cloud. Other

patches of obscuration may also be seen.

The central region shows patches of bright nebulosity^

caused by glowing gas. Obscuration by clouds of dust

and bright nebulosity are often found in the Milky Way

regions in close association.

Photograph by Dr. E. E. Barnard, with the lo-inch

Bruce lens of the Mount Wilson Observatory, 1905,

April 5. Exposure 4^ hours.

l8 LIFE ON OTHER WORLDS

universe, which was originally very compact. An
explosion occurred and the fragments were sent
flying through space in all directions. If such were
the case then, after the lapse of a considerable time,
we would find that the most rapidly moving parts
would be at the greatest distances and that, viewed
from any one of the fragments, all the other frag-
ments would appear to be moving away and with
speeds that were faster the greater the distances.
This suggested explanation may not be the true
one, but it serves to illustrate that there may be
a simple interpretation to facts that at first sight
appear very strange.

The point of interest in the present connection
is that the close proportionality between velocity of
recession and distance affords by far the most accur-
ate and reliable method of estimating the distances
of very remote systems, because the velocity in the
line-of-sight can be estimated from the wave-
lengths of the radiations with good accuracy. It is
found that the velocity in miles a second divided by
1 06 gives the distance in millions of light-years.
Thus, for instance, a distant universe in the con-
stellation of Bootes has been found to be receding
with a velocity of 24,300 miles a second. We can
infer that this nebula is at a distance of about 230
million light-years. Such a distance is beyond the
power of the imagination to conceive. Whilst the
light by which this universe is seen has been
travelling through space, the dinosaurs and flying
reptiles have appeared on the Earth and with the
slow march of evolution have disappeared again.
Mountain ranges have been uplifted and then worn

A PICTURE OF THE UNIVERSE 19

down by erosion. The surface of the Earth has
entirely changed its appearance. And at length,
when the light was Hearing the end of its long
journey, man appeared on the Earth. Such great
distances, though they may surpass our powers of
conception, cannot fail to impress upon us the
vastness of space.

The most distant systems that have been recorded
on long-exposure photographs with the great 100-
inch telescope are at a distance of about 500 million
light-years. Within a sphere of this radius, it is
estimated that there are about one hundred million
universes, the average distance of any universe from
its nearest neighbour being of the order of a million
light-years. At these greatest distances to which
space has as yet been probed, the universes seem tc
be scattered with an approximately uniform distri-
bution ; there is no evidence of any falling off in
density at these extreme distances nor of any
approach to the bounds of space.

Such, in brief, is the picture of the universe pro-
vided by modern astronomical observation. We
see the Earth as a small planet, one member of a
family of planets revolving round the Sun; the Sun,
in turn, is an average star situated somewhat far out
from the centre of a vast system, in which the stars
are numbered by many thousands of millions; there
are many millions of such systems, more or less
similar to each other, peopling space to the farthest
limits to which modern exploration has reached.

Can it be that throughout the vast deeps of space
nowhere but on our own little Earth is life to be
found ? Can astronomy tell us whether life can

2O LIFE ON OTHER WORLDS

exist on any of the other planets belonging to the
solar system; and if it can exist, whether it does
exist ? Is it possible to estimate the likelihood of
life existing somewhere in the universe outside the
solar system ? These are some of the questions
that I am continually being asked. In succeeding
chapters the attempt will be made to answer these
questions, in so far as it is possible for astronomy to
provide answers. But first we must consider what
life is and what tests can be used to decide whether
life is possible or is not possible on any particular
world.

CHAPTER II

THE CONDITIONS FOR
THE EXISTENCE OF LIFE

IN attempting to discuss whether life can exist on
any other world, we come up against the difficulty
that we have no certain knowledge of how life
originated on the Earth. Suppose we could show
that on some other world the conditions were
essentially similar to those on the Earth. Would it
be legitimate to assume that because life has come
into existence on the Earth, there must necessarily
be life also on the other world, though perhaps in
different forms from those with which we are fami-
liar ? If, on the other hand, we could show that the
conditions on another world differed from those on
the Earth to such an extent that no forms of life now
present on the Earth could exist, would it be a
legitimate conclusion that the other world must be
a world devoid of life ? May we not have some
justification for assuming that the forms of life that
now exist on the Earth have developed, through a
slow process of evolution, to suit those conditions
and that, if different conditions were found to pre-
vail elsewhere in the universe, different forms of life
might have evolved ? It is conceivable, for instance,
that we could have beings, the cells of whose bodies
contained silicon, instead of the carbon which is an
essential constituent of our cells and of all other
living cells on the Earth ; and that, because of this
essential difference between the constitution of these

21

22 LIFE ON OTHER WORLDS

cells and the cells of which animal and plant life
on the Earth are built up, they might be able to
exist at temperatures so high that no terrestrial
types of life could survive. To obtain some guid-
ance in endeavouring to answer such questions, we
must consider what biology can teach us about the
nature of life.

All forms of matter, inorganic, organic or living
matter, are built up of the atoms of different ele-
ments. Ninety-two different elements are known
tp the chemist, hydrogen being the lightest and
uranium the heaviest. The atoms of these elements
may be regarded as the bricks from which all matter,
everywhere in the universe, is built up. It might
seem at first sight surprising that the great variety of
substances that we find on the Earth can be built
from so limited a number of different atoms. But
the variety is produced by the great variety of ways
in which the different types of atoms can be com-
bined, just as the richness of our language is the
result of the large number of ways in which the
twenty-six letters of the alphabet can be combined
to form words.

So all the innumerable substances that we find on
the Earth or that we can think of the minerals in
the crust, precious stones, bricks, timber, all living
things, coal, oil and so forth are merely the result
of different combinations of some of the ninety- two
varieties of atoms. But these same atoms are found
also in the Sun, in the stars the most distant as well
as the nearest in the nebulae and in the remote
universe, as well as in the diffuse gaseous matter and
star-dust that is thinly scattered about space be-

CONDITIONS FOR EXISTENCE OF LIFE 23

tween the stars. Conversely , no element is known
to occur in the Sun or the stars that has not been
found on the Earth. It is true that one element was
discovered in the Sun before it had been found on
the Earth; that was the gaseous substance to which
the name helium (from the Greek word for the Sun,
yjXios) was given. Helium was subsequently dis-
covered on the Earth in cleveite and other minerals
containing uranium and was found to be present in
small quantities even in the air that we breathe.

But not merely do we find in the Sun and the
stars the same elements that are found on the Earth,
we find also that the elements that are the most
abundant on the Earth are on the whole the most
abundant in the Sun and the stars, and those that
are least abundant on the Earth are the least
abundant in the Sun and the stars. There are some
exceptions, which are not without significance as we
shall see subsequently, but on the whole the parallel-
ism is very striking and suggests a common origin
from some primordial matter.

How can we detect the presence of this or that
particular element in the Sun or in a distant star ?
The detection is made by analysing the light with a
spectroscope, which breaks the light up into its
different constituents. The light reaching us from
the Sun is highly complex; the atoms of any given
element can vibrate in a number of different ways
and each particular mode of vibration gives rise to
the emission of light of one particular wave-length.
At any instant, some of the atoms will be vibrating
in one particular mode, others in a different mode
and so on. The aggregate of the light radiations

24 LIFE ON OTHER WORLDS

corresponding to these various vibrations of the atom
gives what is called the spectrum of the element, a
series of light- vibrations of definite wave-lengths or
frequencies which is characteristic of the element in
question and is produced by no other element. If
this particular series of vibrations is detected when
the light from a star is analysed , then we may con-
clude" with absolute certainty that the element in
question is present in the star.

The atoms were formerly thought to be small,
hard spheres, the smallest particles of a substance
that could exist by themselves. If an element were
divided into smaller and smaller particles, we
should at length come to a stage when we could sub-
divide no further. We should then have reached
the individual atoms. But modern investigations
of the structure of matter have shown that this
picture is incomplete. The different atoms are
themselves built up of elementary particles, called
protons, neutrons and electrons. The proton car-
ries a positive charge of electricity; the electron
carries an equal negative charge; the neutron, as its
name implies, is electrically neutral. The proton
and the neutron have approximately the same
mass and are much heavier than the electron. A
fourth elementary particle, which has been called
the positron or positive electron, has recently been
discovered; it has the same mass as the negatively
charged electron but a positive charge equal to
that of the proton. The positron is not usually
detected in the presence of matter; it is possible,
though not certain, that the proton is merely a
combination of a neutron and a positron, in which

CONDITIONS FOR EXISTENCE OF LIFE 25

case there would be three fundamental particles-
neutrons, negative electrons and positive electrons.
The atom as a whole is electrically neutral, so
that the total number of protons contained in it
is equal to the total number of negative electrons.

The modern conception of an atom is of a system
containing a nucleus, in which almost all the mass
of the atom is concentrated, composed of protons
and neutrons, whilst outside the nucleus there are
sufficient electrons to make the atom electrically
neutral; we may think of the electrons as describing
orbits around the nucleus, but the system as a whole
is much more complicated than a miniature solar
system. In the solar system, each planet moves in
a definite orbit around the sun; but in the atom,
each electron can move in a number of different
orbits and can jump from one orbit to another.

The simplest atom is the atom of hydrogen,
containing a nucleus of a single proton with a
single electron outside it. It is also necessarily the
lightest of the atoms. The next simplest atom is
the atom of helium; the nucleus of the helium
atom contains two protons and two neutrons and
there are two external electrons. The weight of
this atom is approximately four times the weight of
the atom of hydrogen or, in other words, we say that
helium has an atomic weight of four. So, in succes-
sion, the atoms of the different elements can be built
up, each containing one more external electron than
the preceding and the atomic weight being deter-
mined by the number of protons and neutrons in the
nucleus. It will now be readily understood why
the same atoms are to be found everywhere in the

26 LIFE ON OTHER WORLDS

universe and why we do not find in the Sun, for
instance, a series of atoms entirely distinct from
those that are found on the Earth. The atoms
throughout the universe are built up of the same
fundamental particles, and out of these we can
build up one and only one series of atoms, which
increase in complexity from the lightest elements
to the heaviest.

The atom is something very different from the
hard solid sphere that it was formerly believed to be.
Yet in ordinary chemical processes the atoms of the
different elements retain their identity. The atom
of helium does not split up and form four atoms of
hydrogen. The nucleus of the atom must be split
up in order that we may succeed in transmuting
one element into another. This requires a great
deal of energy, much more than can be furnished
by any chemical action ; the splitting up of atoms
cannot be brought about, therefore, by chemical
action. So although the various atoms are all
built up of protons, neutrons and electrons, we may
still for most purposes think of the atoms as they
were pictured before the new knowledge about the
structure of matter had been gained.

The same atoms, with the same structure, being
found throughout the universe, it necessarily
follows that the chemical laws that they obey will
be the same everywhere, because these laws are the
result of the particular structures and their energy
relationships. The same chemical compounds can
exist under the same conditions anywhere in the
universe. Thus, for instance, two atoms of hydro-
gen and one atom of oxygen can unite to form a

CONDITIONS FOR EXISTENCE OF LIFE 27

stable chemical structure, a molecule of water, and
one molecule of water is precisely similar to every
other molecule. The molecule of a substance is
the smallest part of the substance that can exist
separately. We shall not find that elsewhere in the
universe it will be necessary to have three or four
atoms of hydrogen to combine with one atom of
oxygen to form a stable chemical compound. This
conclusion is of importance for our consideration of
the possibility that living matter may take essenti-
ally different basic forms elsewhere in the universe
from the forms that are found on the Earth. Though
types of vegetable or animal life that are unlike
any types to be found on the Earth may conceiv-
ably occur elsewhere in the universe, the chemical
compounds of which the individual cells are made
up must be such as could exist on the Earth
and it is unlikely that they differ from the com-
pounds of which living matter on the Earth is
built up.

A special role is played in living organisms by
carbon, because it possesses to a far greater extent
than any other element the power of uniting with
itself, as well as with other elements, to build up
single molecules containing very large numbers of
atoms. It is these complex molecules containing
carbon that form the basis of the structure of all
living organisms. The only other element that
possesses the power of building up complex mole-
cules to any great degree is silicon, but the com-
pound molecules which have carbon as a basis are
far more numerous and complex than those which
have silicon as a basis.

28 LIFE ON OTHER WORLDS

Too long a digression into chemical theory would
be necessary to explain why carbon possesses this
unique property. But it is primarily due to the fact
that the atom of carbon is what is called a tetra-
valent atom. Into the formation of every molecule
of any given chemical compound a definite number
of atoms enter. If we consider simple compounds
involving hydrogen, we find that one atom of cer-
tain elements will combine with one atom of
hydrogen; one atom of certain other elements will
combine with two atoms of hydrogen; while one
atom of yet other elements will combine with three
or with four atoms of hydrogen. Thus., for in-
stance, one atom of hydrogen will combine with
one atom of the yellowish poison gas chlorine to
form hydrochloric acid ; chlorine and hydrogen are
called monovalcnt elements. Two atoms of hydro-
gen will combine with one atom of oxygen to
form water; oxygen is called a divalent element.
Three atoms of hydrogen will combine with one
atom of nitrogen to form the pungent gas am-
monia; nitrogen is called a trivalent element.
Four atoms of hydrogen will combine with one
atom of carbon to form the gas methane, familiar
to the miner as the dangerous fire-damp; carbon is
called a tetravalent element.

Two elements that unite with each other, one to
one, have the same valency. Thus, for instance,
the molecule of common salt contains one atom of
sodium and one atom of chlorine. Chlorine is
monovalent and, therefore, sodium must also be
monovalent. The atom of a divalent element may
unite with two atoms of a monovalent element,

CONDITIONS FOR EXISTENCE OF LIFE 2Q

with one atom of each of two monovalent elements,
or with one atom of another divalent element.
The molecule of caustic soda, for instance, consists
of one atom of divalent oxygen linked on one side
with one atom of monovalent sodium and on the
other side with one atom of monovalent hydrogen;
whilst the calcium oxide molecule consists of one
atom of calcium and one atom of oxygen, and
because oxygen is divalent so also must calcium be.

It will be clear that complex molecules cannot
be built up from monovalent atoms ; when two such
atoms unite with each other there are no free
affinities or links left over to which other atoms can
attach themselves. The possibilities are greater but
are still very limited when we consider divalent
elements. The more linkages or affinities that the
atom has, the greater are the varieties of molecules
that can be built up.

This statement is subject to limitation, however.
We have not referred as yet to valencies higher than
four. There are some elements, such as phosphorus
and nitrogen, which are pentavalent, and it might
be thought that these elements would have a greater
power of building up complex molecules than car-
bon has. Such elements, however, tend to behave
as trivalent elements, two of the links joining
together and cancelling each other; we saw above
that in ammonia nitrogen behaves as though it
were trivalent. The maximum power of uniting
with other atoms seems to be reached with the
tetravalent atoms and, amongst the tetravalent
atoms, to the largest degree with carbon. The
carbon atom can have four linkages joined up with

30 LIFE ON OTHER WORLDS

other atoms, when it is said to be saturated, but it
is possible also for two of the four to interplay with
each other, in which case the carbon atom is said
to be unsaturated. An example of an unsaturated
carbon atom is provided by carbon monoxide, the
poisonous constituent in coal gas or in the exhaust
gases from a motor-car engine; the molecule of
carbon monoxide contains one atom of carbon and
one atom of oxygen.

We will consider a few simple compounds* of
carbon to illustrate the facility with which a variety
of compounds can be built up. We start with
methane or marsh-gas, and suppose the atoms of
hydrogen are replaced one by one by atoms of
monovalent chlorine. Methane and the four com-
pounds that can thus be formed may be represented
thus:

H Cl Cl Cl

I I I I

H C H H C H Cl C H Cl C H

H H H Cl

Cl

Cl C Cl

Cl

(H denotes an atom of hydrogen, C an atom of
carbon, Cl an atom of chlorine.)
or by the formulae

l2> CHC1 3 , CC1 4

CONDITIONS FOR EXISTENCE OF LIFE 31

In the first representation, by what chemists term
a structural formula, the relationship of the several
atoms to one another in each molecule of the sub-
stance is shown. The second group of formulae
merely indicates the chemical constitution of the
molecules, without giving any information about
their structure. The structural formulae, giving
an indication of the structure of the molecules, are
much the more informative.

The first substitution of one atom of chlorine gives
the substance called methyl chloride. The group
(CH 3 ), in which the carbon atom has one free link,
behaves like a simple monovalent atom and can
take the place of monovalent atoms in other com-
pounds, so forming compounds of greater com-
plexity; it is called the methyl group. The next
compound is di-chlor-methane. By the substitu-
tion of a further atom of chlorine we obtain tri-
chlor-methane, which is the chemical name for the
anaesthetic, chloroform. When all the hydrogen
has been replaced by chlorine we obtain carbon
tetra-chloride, an important organic solvent, parti-
cularly of fatty or greasy substances, which has the
advantage of not being highly inflammable. It is
therefore used extensively in dry cleaning processes,
being safer than inflammable solvents such as
petrol or benzene.

The organic substances that are built up by living
plants and animals can be classified into three
broad divisions, the carbohydrates, the fats and the
proteins. We will illustrate briefly how such sub-
stances can be built up in progressively increasing
complexity. We start with the simplest of the

PLATE 4

THE MOON: THE APENNINES AND ARCHIMEDES

The Apennine Mountains form the greatest range of
mountains on the Moon, being nearly 640 miles in
length and reaching a maximum height of 21,000 feet.
They rise gradually on the S.W, side, but fall away
steeply, with great precipices, on the N.W. (In the
plate, North is at the top and West at the right.)

North and east of the Apennines lies the great lunar
plain, called Mare Imbrium, or Sea of Storms. Ripple
marks on this plain mark the limits of successive flows
of lava.

In the centre of the upper portion of the photograph
is the ring-mountain or lunar crater, known as Archi-
medes. It is 50 miles across. The highest part of the
mountain-ring is 7,400 feet above the interior, whose
level is 650 feet below that of the surrounding surface.

The two craters near the top right-hand corner are
known as Aristillus (upper) and Autolycus (lower).

Photographed by Dr. J. H. Moore and Mr. J. F.
Chappell, with the 36-inch refractor, Lick Observatory,
1937, October 26.

PLATE 5

THE MOON: REGION OF TYCHO AND CLAVIUS

The photograph shows part of the southern portion of
the Moon's surface, illustrating its extreme ruggedness.
The whole of this region is honeycombed with craters of
all sizes from the smallest to the largest, many craters
being contained within, or overlapping, the walls of
other craters.

Immediately to the left of the centre of the photograph
is Tycho, the most perfect specimen of a lunar crater.
It is 54 miles wide and 17,000 feet deep, so that Mt.
Blanc, if placed inside it, would not reach the top.
The central peak, whose shadow can be seen on the
floor of the crater, is 5,500 feet in height. From this
crater extends a great ray system, which can be seen
near full moon; some of the rays extend over thousands
of miles, passing across mountains and valleys.

The large crater at the bottom of the photograph is
Glavius. It has a diameter of 142 miles. There are
numerous small craters on and within its ring.

Photographed by Dr. J. H. Moore and Mr. J. F.
Chappell, with the 36-inch refractor, Lick Observatory,
1937, October 26.

34 LIFE ON OTHER WORLDS

sugars, whose basis is six carbon atoms in the form
of a chain, with links to hydrogen and to the group
called hydroxyl, consisting of an atom of oxygen
and an atom of hydrogen ( O H), which in a
similar way to the methyl group acts as a mono-
valent atom. The diagrammatic or structural
formula is:

H H H H H H

1 I I I I I
rj n n n r; p _ Q

\^i \~J \^t \^4 \^J \_J \ /

I I I I I

OH OH OH OH OH

It may be noted in passing that groups containing
six carbon atoms occur very frequently in carbon
compounds and seem to possess great stability.

A more complex sugar can be obtained by com-
bining two such molecules. If we imagine one of
the hydrogen atoms removed from the first molecule
and one of the hydroxyl groups removed from the
second, we can join up the resulting free linkage to
form cane sugar, a disaccharide, whilst the hydroxyl
group and the hydrogen atom, which have been set
ifree, join up to form a molecule of water, H O H.
This process can be repeated and, theoretically, we
can build up in this way molecules of sugars or
starches of any desired degree of complexity. The
starches found in living plants contain thirty to
forty of such groups. There should in practice
presumably be a limit, conditioned by the energy
relationships, beyond which further reduplication
would not be possible, the molecules becoming
unstable and breaking down. But the process

CONDITIONS FOR EXISTENCE OF LIFE 35

illustrates the adaptability of the carbon atom as the
basis for large molecular aggregations. The sugars
and starches, with this general type of structure,
form the carbohydrates of living matter.

The basic structure of fats of living cells can be
explained by starting with glycerine ; the molecules
of this substance consist of a chain of three carbon
atoms in which each carbon atom is united to a
hydrogen atom and a hydroxyl group, giving the
structure :

H H H

I i I
H C C C H

I I I
OH OH OH

When each hydroxyl group is replaced by a
molecule of a fatty acid, a fat is formed. A
hydrogen atom from the fatty acid molecule
combines in this process with the hydroxyl group
to form water.

The fatty acid is usually built up on a basis of
chains of six carbon atoms; the following structural
formula represents a simple fatty acid:

H H H H H

I I I I I
H G C C C G G = O

I I I I I I
H H H H H OH

This formula may be compared with the formula
of the simple sugar given above. It is the group

36 LIFE ON OTHER WORLDS

on the right-hand side

group, which confers the acid properties.

If such a molecule combines with the glycerine
molecule, the hydrogen of the (OH) acid group
combines with one of the (OH) groups in the
glycerine molecule to form water, the two free
linkages being joined. When the three (OH) groups
in the glycerine molecule have been replaced in
this way by three fatty acid molecules, each of
which loses the hydrogen atom of its acidic group,
a fat is obtained. The fatty acids that are formed
in nature in living cells are usually of a more
complex nature than this, being derived from the
union of three of the six-carbon chains, only one
of which possesses the acid carboxyl group. There
is then a row of eighteen carbon atoms. Three
such complex molecules react with the glycerine
molecules to form the large molecule of a fat. It
will be realised that the process of carbon-chain
building can be considerably extended with the
formation of large and very complex molecules,
all of which possess, however, the same general
characteristics.

To explain the basis of the structure of the third
group of substances found in living matter, the
proteins, we again start with the simple fatty acid,
based on the six-carbon chain, whose formula was
given above. If the hydrogen atom at the end of

the chain is replaced by the amino group, N<^TT,

obtained from an ammonia molecule that has lost
one of its atoms of hydrogen, and which behaves

CONDITIONS FOR EXISTENCE OF LIFE 37

as a monovalent atom, a substance is obtained that
has the formula:

H H H H H H

N G C C C C C = O

I ! I I I I I

H H H H H H OH

(N denotes an atom of nitrogen)

Such a substance is one of a large group of
substances that are called amino acids. It will be
noted that it contains at one end the carboxyl
group, which confers acid properties, whilst
at the other end it contains the amino group,
which confers alkaline or basic properties. It
may be mentioned that ammonia, from which
the amino group is derived, is a strong base. The
amino acids are, therefore, acid in one part and basic
in another and according to the circumstances they
can act either as an acid or as a base. The amino
acids form the basis of all the proteins. Many of
these have an extremely complex structure, with
molecular weights running into many thousands.
The acid portion of one amino acid has a chemical
attraction for the basic portion of another and this
enables them to unite to form a more complex
amino acid. The hydrogen atoms may be replaced
by many diverse groups, of a more or less com-
plicated nature, without affecting the characteristic
amino-acid properties. There may be two amino
groups instead of one, forming a di-amino acid;
sometimes also there are two acid groups. The
peculiar property of the carbon atom of forming

38 LIFE ON OTHER WORLDS

chains makes possible a vast number of different
combinations, with the result that an almost infinite
variety of structures can be obtained from the four
elements hydrogen, carbon, nitrogen and oxygen.

An important process in nature is the building
up of carbohydrate into fat in living structures ; the
process is important because the fat provides a
larger store of energy. The energy that is stored
in this way is derived from sunlight. Reference to
the structural formulae given above shows that the
carbohydrates contain the repeated structure

H H

I . I

C , whilst the fats contain the repeated C

| structure .... |

OH H

The action of sunlight is to split up one of the OH
groups in the carbohydrate and to replace it by a
hydrogen atom, the oxygen being given off in the
process and energy from the sunlight being utilised.
This is essentially the process that the chemist calls
reduction. It can be reversed; the organic sub-
stance then becomes oxidised and energy is released,
which is available for vital processes.

We have seen that the large molecules built up
around long chains of carbon atoms form the basis
of living matter. A further stage of complexity is
produced by the tendency for a number of these
large molecules to unite to form molecular aggre-
gations. In such molecular unions, each molecule
behaves as a single atom.

The separate molecules in the aggregation are
feebly held together without true atomic union;

CONDITIONS FOR EXISTENCE OF LIFE 39

very little evolution of energy is involved in the
formation of such aggregations, which are in a
somewhat unstable condition. They exist in a state
of delicate balance. This is the distinguishing
feature of the state of matter that is termed the
colloidal state. The study of matter in this state has
become an important branch of chemistry. It has
proved of particular significance in biochemistry
the chemistry of living matter because much of
the material of which living cells are constructed
exists in this state. The colloidal substances may
be either active in solution, or they may be inactive
masses which have been formed from the dissolved
living colloids and thrown out of solution. The
latter process gives rise to the membranes that sur-
round the separate cells and divide one from
another. It may be illustrated by the formation of
a skin on the surface of warm milk. The chief
colloid of the milk accumulates on the surface and
the molecular aggregations join together to form a
close network or film.

To consider the special properties of matter in
the colloidal state and to discuss the reasons why
the colloidal state plays so large a part in vital
processes would be outside the scope of this book.
The important point to emphasise for our present
purpose is the tendency for the occurrence in living
matter of large molecular units in a state of a
delicate balance of equilibrium.

The preceding discussion may be summed up as
follows: the same atoms that occur on the Earth
are to be found in the remotest parts of the Universe.
The same chemical laws necessarily prevail through-

40 LIFE ON OTHER WORLDS

out the Universe. The great variety of substances
that are necessary to form living matter is made
possible by the peculiar power of the carbon atom
of uniting with other atoms. The chemistry of
carbon is, therefore, of great importance in the study
of living matter and wherever in the Universe
living matter may occur it must be dependent upon
the special properties of the carbon atoms. The
formation of large molecules with a chainlike
structure and of feebly stable molecular groups
must be possible, it would seem, if living matter
is to exist.

These conclusions can serve as a guide in con-
sidering the conditions that are necessary for the
existence of life to be possible. The first requisite
is that the temperature should be neither too high
nor too low. Every chemical compound can be
split up or dissociated by raising its temperature
sufficiently. The ease with which various sub-
stances can be broken up by heating differs con-
siderably. In the hottest stars no compounds at
all can be detected; matter can exist only in the
atomic state. In the Sun, whose temperature is
about 6,000 C., a few very simple compounds
that strongly resist dissociation are found, such as
silicon fluoride and cyanogen. The temperature of
sunspots is about a thousand degrees lower than the
temperature of the rest of the Sun and this lower
temperature enables other simple compounds to be
present in the spots, which cannot exist at the higher
temperature of the rest of the Sun; amongst such
compounds are titanium oxide, boron oxide and
the hydrides of magnesium and calcium. For the

CONDITIONS FOR EXISTENCE OF LIFE 41

cool red stars, with temperatures of the ordef of
3,000 G., bands due to compounds become more
prominent in the 'spectra; but the compounds are
simple compounds, titanium and zirconium oxides,
cyanogen and a few other simple carbon compounds.
In general, the more complex the structure of the
molecules of a substance, the more readily they are
broken up when the temperature is raised. We
have seen that the molecules of which living mattes
is composed are extremely complex. For this
reason they are also very fragile and have but small
power to withstand being broken up when the
temperature is increased. All forms of life that we
know are, in fact, very sensitive to high temperature.
The surest way to kill any form of life is to subject
it to a high temperature. For this reason milk is
sterilised by the process of pasteurisation; no milk
can be sold as pasteurised unless it has been main-
tained at a temperature between 145 F. and 150
F. for not less than 30 minutes. It is found that
this treatment ensures the destruction of any
disease-producing organisms, such as the tuber-
culosis bacilli, which may be present in the milk.
For the same reasons, water of doubtful purity can
be 'made safe for drinking by being well boiled.
Bearing in mind that living matter, wherever it may
occur in the Universe, must be complex in structure,
we may reasonably conclude that wherever tem-
peratures are sufficiently high to destroy any form
of life that occurs on the Earth there can be little
expectation that life will be found. We may note,
moreover, that the higher forms of life are less
resistant to heat than the simple organisms. The

42 LIFE ON OTHER WORLDS

temperature might be such that the simplest types
of life could exist, though any development of
higher and more complex types of life could not
occur. Without attempting to lay down any hard
and fast limit, it does not seem probable that
complex living structures can be expected wherever
the temperature is much in excess of about 150 F.
Though most forms of life are unable to survive
at very low temperatures, it is known that some can
withstand extreme cold for long periods. Low
temperatures, unlike high temperatures, do not
break up chemical compounds. But when life is
not actually destroyed by very low temperatures it
becomes latent, as it were. There seems to be a
state of suspended animation, in which all vital
processes are suspended until the temperature is
raised. We cannot believe that, if these conditions
prevailed on any other world, life could develop, for
how can there be any development if vital processes
are suspended ? The reason why low temperatures
are inimical to life is evident. The development of
vital processes requires energy. On the Earth, the
energy of all living things is ultimately dependent
upon green plants, which in turn are dependent for
their energy upon radiation from the Sun. On a
planet in some other solar system, vital processes
must similarly be ultimately dependent upon
energy received from the parent Sun. The temp-
erature of any such world is conditioned by the
energy that it receives from its parent Sun; a low
temperature implies that it receives but a small
supply of energy. If the temperature is so low that
there is not sufficient energy available for vital

CONDITIONS FOR EXISTENCE OF LIFE 43

processes, there can be no life. We must conclude,
therefore, that if on any world we find either a high
or a low temperature, it is extremely improbable
that life in any form could exist on it.

It will be clear that the restriction to a moderate
temperature enables all the stars to be at once
ruled out of consideration as possible homes of life.
For the temperatures of the stars are so high that
even on the coolest of them only a few of the simplest
compounds are to be found. All the complicated
molecules that are present in every type of living
organism are broken up by heat at temperatures far
below those that prevail in the coolest stars. Our
search for life elsewhere in the universe must, there-
fore, be restricted to the planetary bodies, which
have much lower temperatures than the stars. It
may be mentioned that, on this subject, Sir William
Herschel, whose conclusions about the structure of
the Universe were far in advance of his time, held
views that were strangely erroneous. He believed
that the Sun was a cold, dark body, covered with a
layer of fiery clouds. He thought that it was " most
probably also inhabited, like the rest of the planets,
by beings whose organs are adapted to the peculiar
circumstances of that vast globe. "

Given satisfactory conditions of temperature,
some further clues as to the requirements for the
existence of plant or animal life can be obtained
from consideration of the requirements for life on
the Earth. The green plant builds up carbohydrates
in the following way: carbon dioxide is absorbed
from the air by the plant and enters into union with
he water in the plant, forming carbonic acid

44 LIFE ON OTHER WORLDS

(H 2 O -f CO 2 ), which may be represented by the
OH

I

formula HO G = O. Under the action of sun-
light, one of the OH groups is split up and replaced
by hydrogen, the oxygen being given off by the plant
to the air. Sunlight provides the energy that is
needed for this transformation to be able to take place,
but it is the green colouring matter in the plant,
called chlorophyll, which makes the transformation
possible. For this reason, chlorophyll is called a
photocatalyst; it enables the transformation to occur
under the action of light, taking some complex
part itself in the transformation, though it remains
unchanged when the transformation is completed.
By this action, a substance with the formula
H

I
HO G = O, known as formic acid, is produced;

it contains the characteristic carbohydrate group-
ing, H C OH. When the formic acid is in turn
reduced, and six of the resulting groups unite to
form a six-carbon chain, a carbohydrate is ob-
tained; the formation of more complex carbo-
hydrates and of fats, in the way already explained,
follows naturally.

There are many lower plants that obtain all their
carbon in the dark, without the action of sunlight,
by the reduction of carbon dioxide and then synthe-
sise their organic constituents from that source. In
such cases, chemical energy provides the supply of
energy that is needed, instead of the luminous
energy from sunlight. Whichever process is followed,

CONDITIONS FOR EXISTENCE Ol LIFE 45

a supply of carbon dioxide is essential, from which
the plant can obtain its carbon. It would seem,
therefore, that plant life is dependent upon carbon
dioxide being available, though the carbon dioxide
need not necessarily be present in large amount,
The proportion of carbon dioxide contained in the
atmosphere of the Earth is only between three and
four parts in ten thousand; this small proportion
supplies the needs of the extensive plant life on the
Earth.

The supply of carbon dioxide in the Earth's
atmosphere is maintained by the process of com-
bustion. The combustion of coal consists essenti-
ally in the combination of carbon and oxygen, with
the production of carbon dioxide. If a supply of
oxygen is not available the coal will not burn. The
process of burning coal, considered as a chemical
action, is, therefore, merely the oxidation of carbon.
Heat is given out when coal burns; this means
merely that the chemical process of the oxidation
of carbon into carbon dioxide is accompanied by
the production of heat. In other words, the
oxidation of carbon provides a supply of energy.

The body of every living animal is continually at
work, and is therefore using up energy. This
energy must somehow be supplied to it. In
general, the supply of energy is obtained by com-
bustion or, in other words, by the oxidation of
carbon. This chemical process can occur, it need
hardly be explained, without any visible flame; the
combustion of a given quantity of carbon will
provide the same amount of energy whether it takes
place slowly, as in the human body, or rapidly and

46 LIFE ON OTHER WORLDS

with the production of flame, as when coal is burned.

If energy is to be provided by the process of com-
bustion, a supply of oxygen is necessary, whether
the organism lives in water or out of it. The
organism must, therefore, be provided with some
means by which it can take oxygen from the atmo-
sphere or from the water. In the lower forms of life
the oxygen is absorbed through the skin and the
carbonic acid or carbon dioxide, the end-product of
combustion, is got rid of through the skin. In the
course of evolution, special organs of breathing
have been developed in many types of life; the gills
of fishes, the respiring membranes of spiders and the
lungs of human beings and many other animals are
instances of such organs which, by increasing the
area of the breathing surface, enable sufficient
oxygen to be taken in to supply, through the process
of combustion, the energy requirements.

Whenever, therefore, the supply of energy for the
maintenance of vital processes is provided by com-
bustion, an adequate supply of oxygen is necessary
for the continuance of life. But is it possible that
energy can be provided in any other way ? There
is one other source of energy, which is utilised by
some organisms, viz. the process of fermentation.

The molecule of sugar, represented diagram-
matically on p. 34, can be broken up into two
molecules of alcohol :

H H

I I
H G C OH

I I
H H

CONDITIONS FOR EXISTENCE OF LIFE 47

and two molecules of carbon dioxide. The chemical
equation representing the transformation is as
follows :

C 6 H 12 6 = 2 C 2 H 6 + 2 C0 2

The sugar is broken up, alcohol being produced
with evolution of carbon dioxide. This is merely
the ordinary process of fermentation, which occurs,
for instance, when wine is formed from grape-juice.
The act of fermentation is accompanied by the
evolution of heat, this heat representing the energy
which can be made use of by any organism that
relies upon fermentation for its supply of energy.
The chemical change called fermentation, repre-
sented by the above equation, is brought about by
yeast or other forms of cell life, which supply
enzymes or transformers, whose presence makes
possible a chemical change that would not other-
wise take place.

The process of fermentation is not a very efficient
method of providing energy; it is far inferior in
efficiency to the process of combustion. It is conse-
quently not well adapted as a general means of
providing energy required for the maintenance of
vital processes. It is the method employed, for
instance, by certain parasitic organisms that live in
the intestines, where a plentiful supply of starch or
sugar is available. Such organisms do not need to
provide energy for the maintenance of their own
heat, because they live a sheltered existence under
conditions of uniform temperature. They are able
to live without any supply of oxygen, though they
are dependent for their continued existence on a

48 LIFE ON OTHER WORLDS

host who could not live without a supply of oxygen;
they are consequently dependent in an indirect
manner upon a supply of oxygen.

It seems reasonable to conclude that animal life is
normally dependent upon a supply of oxygen, but
that under exceptional circumstances it can exist
without oxygen; it must then be able to obtain a
supply of energy by the process of fermentation.

A further condition for the existence of life seems
to be the presence of water, either in the liquid form
or as water vapour. Neither seeds nor spores will
germinate in absolutely dry soil; when life is not
actually destroyed by the absence of moisture it
becomes latent and no development ensues. Water
is an essential constituent of the tissues of both
animal and vegetable life, because a cell needs a
certain amount of water to carry on its life. It is by
imbibing water containing the chemical substances
in which they feed that cells are enabled to grow
and to multiply by continual division. In particu-
lar, we find that egg and sperm cells are specially
sensitive cells, which cannot withstand much
desiccation; in order that fertilisation may take
place these cells must meet either in water or in a
damp atmosphere such as is provided by the female
sex ducts. In many animals that live in water, such
as fishes and frogs, fertilisation takes place outside
the body; in most animals that live on land, on the
other hand, fertilisation takes place internally,
within the female sex ducts. It does not seem
probable that in the absence of moisture any form
of life could develop.

There are many gases that have a marked toxic

CONDITIONS FOR EXISTENCE OF LIFE 49

action on living organisms, such as ammonia,
chlorine, carbon monoxide and sulphuretted hydro-
gen. Though their presence in the atmosphere of
any world might not necessarily prove conclusively
that there could be no life on it, it would provide
strong evidence against its probability. We shall
find that poisonous gases are prominent constitu-
ents of the atmosphere of some of the planets.

The considerations summarised in this chapter
should provide a reliable guide to the possibility
or the impossibility of the existence of living organ-
isms on another world, provided that we can
obtain sufficient information about the conditions
that prevail on it. If we can show that the con-
ditions are favourable for life, it may not necessarily
follow that there must be life. What is certain is
that if suitable conditions exist, if there is an ade-
quate supply of energy and if there is a suitable
transformer for that energy, which can turn it into
the chemical energy of carbon compounds, then
the complex organic substances which form the
basis of living cells not only can arise but will arise.
How the step from these complex organic sub-
stances to the simple living cell is made is not known.
Nevertheless, it seems reasonable to suppose that
whenever in the Universe the proper conditions
arise, life must inevitably come into existence.
This is the view that is generally accepted by
biologists.

It has been supposed by various people at one
time or another that life did not arise on the Earth
spontaneously, but that it was carried to the Earth
by cosmic dust particles or by material particles

50 LIFE ON OTHER WORLDS

from some other world where life was already in
existence. In some forms of this hypothesis, it is
suggested that life was once created on one world
and that it has ever since been disseminated from
this original source. Such a theory does riot bring
us any nearer to understanding how life has come
into being and merely removes the problem from
the possibility of investigation by shifting it to
some remote world. Helmholtz adopted a rather
different form of the hypothesis and queried whether
life had ever arisen; he suggested that life might be
as old as matter and that the germs of life passed
from one world to another and developed wherever
they found suitable soil. All such hypotheses
appear to be inherently improbable and they leave
the solution of the problem of the origin of life as
remote as ever.

If we accept the view that life on the Earth
arose as the result of the operation of certain
natural causes and that life will arise or has arisen
on any other world where the proper conditions
prevail, it does not follow that life will develop or
that it has developed on such a world along the
identical lines that it has followed on the Earth.
The various forms of life that have appeared in
succession on the Earth have no doubt resulted
from the particular conditions, themselves slowly
changing, that have prevailed on the Earth. If
we consider the geological history of the Earth
the cooling down of the material in its initial
gaseous condition until liquefaction took place,
followed by the formation of a solid crust as further
cooling ensued ; the successive stages of crumbling

CONDITIONS FOR EXISTENCE OF LIFE 5!

and folding of the surface with the formation of
mountain ranges, as the solid crust adjusted itself
to a cooling and shrinking interior; the condensation
of water vapour to form the oceans ; the formation of
sedimentary deposits beneath the oceans, through
the denuding action of rain, streams and rivers,
and their subsequent upheaval to form new land
areas; the changes of climate shown by alternating
ice-ages and glacial periods it will be realised that
life on the Earth, in the various forms that now
exist, has been conditioned by a complex series of
changes. The general sequence of events may have
been similar on another world, but their details
must inevitably have differed considerably. Differ-
ences in temperature, in the constitution of the at-
mosphere, in the proportions of land and water
and in their distribution, and in the general
accompanying phenomena must have profoundly
influenced the course of evolution. The degree
of adaptation to their environment shown by many
animals provides evidence that mere environment
has had a considerable effect on the development
of life. It is reasonable to suppose, therefore, that
life on any other world will have developed along
forms that are entirely different from any with
which we are familiar and that are possibly beyond
our conception. Divergences, which may have
been small in the most primitive types of life, would
tend to become more and more accentuated in the
slow course of evolution. Whether, on any other
world where life may have arisen, intelligent beings
the counterpart of man have evolved must be a
question for mere surmise. It must remain outside

52 LIFE ON OTHER WORLDS

the scope of this discussion. The most that we are
justified in attempting is a discussion of the suita-
bility of the various planets of the solar system as a
home of life and of the possibility that other
planetary systems may exist amongst which there
may also be some with conditions favourable for
life.

CHAPTER III

METHODS OF
INVESTIGATION

BEFORE considering what information can be
obtained about the conditions that prevail on the
various planets in the solar system, we must
describe the methods by which some information
can be gained about the extent and nature of their
atmospheres and about the temperatures at their
surfaces.

Simple considerations can tell us whether or
not any planet may be expected to possess an
atmosphere. Before looking at the planet in the
telescope, we shall know what observation is likely
to reveal.

Let us consider what an atmosphere really con-
sists of. A gas is composed of an aggregation of
molecules which are in a state of perpetual motion.
The molecules move rapidly about in all directions,
describing straight paths with uniform velocity,
except when they collide with other molecules.
If the gas is contained in a closed vessel, the con-
tinual bombardment of the walls of the vessel by
the molecules that hit them produces an integrated
effect which we call the pressure of the gas. If
we double the quantity of gas in the vessel, the
number of collisions of the molecules with the walls
in each second is doubled; the pressure is therefore
doubled. That is why the pressure in a motor
tyre increases as we pump air into it.

53

54 LIFE ON OTHER WORLDS

If we think of a simple gas, all of whose
molecules are of the same kind, at any given
instant some molecules will be moving faster than
others.

The speeds of some molecules will be reduced
when a collision occurs ; the speeds of other mole-
cules will be increased. It is impossible to study in
detail the motions of the individual molecules bvit
the statistical distribution of the velocities amongst
the complete aggregation of molecules can be
investigated. The mathematical investigation of
the statistical properties of the molecular assem-
blage forms the subject called the kinetic theory
of gases. For a given temperature, the average
velocity of all the molecules remains statistically
constant. The number of fast-moving molecules
with velocities greater than a value that is much in
excess of the average velocity falls off very rapidly
as this value increases; but there are definite,
though small, proportions of molecules with
speeds of 10, 20 or even 100 times the average
speed.

The velocities of the molecules depend upon the
temperature of the gas. The average velocity is
proportional to the square root of the temperature
measured from absolute zero (273 C.). At
the absolute zero of temperature, the molecules
have no velocity. They are all brought to rest.
No lower temperature than this can be imagined.
Physicists have been able to reach temperatures
within a fraction of a degree of the absolute zero,
but the zero itself has never been reached. The
average velocity at the temperature of boiling

METHODS OF INVESTIGATION 55

water (iooG.) is, for instance, 17 per cent,
greater than the average velocity at the freezing
point of water (o C.). By means of this relation-
ship, the average velocity at any temperature can
readily be found if its value at some given temper-
ature is known.

In a mixture of gases, the molecules of different
kinds move at different speeds. The lighter the
molecules the faster they move on the average.
There is a simple law, known as the law of equi-
partition of energy, which governs the distribution
of velocities amongst the different kinds of mole-
cules. The law of equipartition of energy states
that in a mixture of gases the average energy of
each kind of molecule is the same. Since the
energy of a molecule is proportional to its molecular
weight multiplied by the square of its velocity, it
follows that the average speed of each kind of
molecule is inversely proportional to the square
root of its molecular weight. In a mixture of
oxygen and hydrogen, for instance, the average
speed of the molecules of oxygen will only be one-
quarter of that of the molecules of hydrogen,
because oxygen is sixteen times as heavy as hydro-
gen.

It is more convenient, in general, to use a
velocity which is such that the pressure exerted by
the gas is the same as if all the molecules were
moving with this particular speed. It is slightly
different from the average speed, being about
nine per cent, larger. This velocity is tabulated
for several gases, at the temperature of o C.
(32 F.).

56 LIFE ON OTHER WORLDS

Miles a second.

Hydrogen . . . . .1*15

Helium ...... 0-82

Water vapour . . . . .0-38

Nitrogen . . . . .0*31

Oxygen ...... 0*29

Carbon dioxide . . . .0-25

In a cubic inch of air at normal temperature and
pressure (o C. and 760 mm. of mercury) the
number of molecules is about 500 million billion.
The figures just given indicate that these molecules
are moving with high speeds. It is evident that
no molecule will move very far before it collides
with another molecule. The average distance
travelled by a molecule before it undergoes a
collision is only about the two hundred thousandth
part of an inch. The average distance traversed
by a molecule between collisions is inversely pro-
portional to the number of molecules in a unit
volume and therefore becomes greater as the pres-
sure is decreased. In a vacuum tube in which the
pressure is one-tenth of a millimetre of mercury,
the average distance between collisions is about
one twenty-fifth of an inch. The average interval
of time between successive collisions becomes
correspondingly greater as the pressure is dimin-
ished.

The density of the gaseous atmosphere of the
Earth or of another planet decreases with increasing
distance from the surface of the planet. Near the
upper limit of the atmosphere, where the density
is very low, the molecules will travel for a con-
siderable distance between collisions. If a mole-
cule in this region happens to rebound after a

METHODS OF INVESTIGATION 57

collision in an outward direction and with a speed
much greater than the average speed, there is the
possibility that it may escape into outer space,
provided that it does not come into collision with
any other molecule.

In order that any particle, whether large or
small, may be able to escape altogether in this
way it is necessary that its velocity should
exceed a certain critical value called the velocity
of escape. The velocity of escape plays a very
important role in our consideration of planetary
atmospheres.

A gas possesses the property of spreading
throughout the whole of any space in which it is
placed. If, for instance, we place a small sealed
flask containing gas in an evacuated chamber and
then break the flask, the gas will at once spread
throughout the whole of the chamber. This is the
natural result of the movements of the mole-
cules, which travel in straight paths until they
collide with other molecules or with the walls of
the containing vessel. Why, then, does not the
atmosphere of the Earth rapidly dissipate away
into space ? Why do the outermost molecules not
fly away ? The reason is that the atmosphere is
held bound by the gravitational pull of the Earth.
The same force that makes the apple fall from the
tree to the ground holds the air captive and pre-
vents it spreading out into space.

Let us suppose that a stone is dropped from a
certain height to the ground. It falls with an
accelerated velocity because of the attracting force
of gravity. We can imagine this motion to be

58 LIFE ON OTHER WORLDS

reversed at each point of its path. If the stone is
projected upwards with a velocity equal to that
with which it hit the ground, its velocity will
progressively decrease; at any distance above the
ground, its upward velocity will be equal to its
downward velocity at the corresponding distance
when it was falling. The stone will come to rest
instantaneously at a height equal to that from which
it was previously dropped and it will then com-
mence to fall to the ground again.

We now imagine the stone to be dropped from
an infinitely great height, and we suppose the
Earth to be isolated in space so that we need
not concern ourselves with the gravitational attrac-
tion of any other bodies. The stone will fall
towards the Earth with a gradually increasing
velocity. It will reach the ground with a certain
velocity, V, which will have a finite value, although
the stone has fallen from an infinitely great distance.
V is, in fact, given by the formula

V 2 = zGMfa

where G is the value of the constant of gravitation,
M is the mass of the Earth and a is its radius.

If the stone is projected upwards with the
velocity V, it will reach an infinitely great distance
before it comes to rest; if it is projected with a
velocity less than V, it will eventually come to rest
and then fall back to Earth, because any velocity
less than V corresponds to the velocity acquired in
falling to the ground from a height that is finite.
It follows that the stone can only get completely
away from the earth if its initial velocity is equal to.

METHODS OF INVESTIGATION 59

or greater than V. It is for this reason that V is
called the velocity of escape.

We can determine the velocity necessary for any
body to escape from the Earth by substituting the
values for the constant of gravitation, and for the
mass and radius of the Earth. In this way we
find that the velocity of escape from the Earth is
7-1 miles a second. 1

Returning to the consideration of the outer
layers of the atmosphere, not a single molecule
can possibly escape into outer space unless its
velocity exceeds the escape velocity. But whenever
a molecule rebounds away from the Earth with a
speed greater than the escape velocity, it will
escape from the Earth's gravitation, provided that
it does not collide with any other molecule. There
must inevitably be such a loss of the faster-moving
molecules from the outer layers of the atmosphere.

Comparing the velocity of escape from the Earth
with the average speeds of different types of mole-
cules given above, it appears that a molecule of
hydrogen must have rather more than six times its
average speed to be able to escape, whereas a
molecule of oxygen must have nearly twenty-five
times its average speed. It is therefore much
easier for hydrogen to escape than for oxygen.

1 For those who are interested in verifying this value, the data are :

Constant of gravitation, G = 6-67 io 8
Mass of Earth (in grams), M = 5-97 x io 27 gm.
Radius of Earth (in cms.), a = 6-37 x io 8 cm.
With these values we find
V= 1-13 X io 8 cm./sec. = 11-3 km./scc.

= 7- 1 miles a second.

6O LIFE ON OTHER WORLDS

But this statement is merely qualitative. Under
any given conditions at what rate will the loss of
atmosphere occur ?

This is a problem that is capable of treatment by
the mathematical principles of the kinetic theory of
gases. The necessary calculations were made some
years ago by Sir James Jeans. He found that if
the velocity of escape is four times the average
molecular velocity, the atmosphere would be
practically completely lost in fifty thousand years;
if the velocity of escape is four and a half times the
average molecular velocity, the atmosphere would
be lost in thirty million years; whilst if the velocity
of escape is five times the average molecular
velocity, twenty-five thousand million years would
be required for the loss to be almost complete.

The rate at which an atmosphere is lost is there-
fore conditioned in a very critical manner by the
ratio between the velocity of escape and the average
velocity of the molecules. If this ratio is 4, the rate
of loss is very rapid, remembering that the age of
the Earth is of the order of three or four thousand
million years; if the ratio is 5, the rate of loss is so
slow that we can regard the atmosphere as practi-
cally immune from loss. If, therefore, we know the
velocity of escape from any planet (which is de-
pendent upon a knowledge of the mass and radius
of the planet) and the averager velocity of the
molecules (which is determined by the molecular
weight and the temperature), we can estimate
with considerable accuracy whether the planet is
likely to have retained its original atmosphere
almost in its entirety, or to have lost a substantial

METHODS OF INVESTIGATION

6l

portion of its atmosphere, or to have lost essentially
the whole of its atmosphere.

In the following table are given, for the Sun,
Moon and planets, the radii and masses in terms of
the corresponding quantities for the Earth as units,
together with the velocities of escape in miles a
second.

Radius

Mass

Velocity of es

(Earth - i).

(Earth = i).

(miles/sec.

Sun .

lOQ-I

332,100

392

Mercury

o-39

0*044

2-4

Venus

0-97

0-82

<3'5

Earth

I-OO

I -OO

7-1

Moon

0-27

O-OI23

i'5

Mars

o-53

0-108

3'2

Jupiter

io-95

3i7-i

38

Saturn

9-02

94'9

23

Uranus

4-00

14-65

H

Neptune

3*92

17-16

'5

Pluto

? O-IO

? o-oi

? 2'2

Mere inspection of the figures for the velocities of
escape, given in the last column, suggests that the
large planets, Jupiter, Saturn, Uranus and Nep-
tune, may be expected to have atmospheres that are
much more extensive than the Earth's atmosphere;
that Venus may be expected to have an atmosphere
comparable with that of the Earth; that Mars may
be expected to have an atmosphere considerably
thinner than that of the Earth and that Mercury
and the Moon may be expected to have little or no
atmosphere. The extent to which these expecta-
tions are confirmed with be considered in subse-
quent chapters.

62 LIFE ON OTHER WORLDS

When a planet has been proved to have an
atmosphere, we naturally wish to find out as much
as possible about the composition of the atmosphere.
For this purpose, we must have recourse to the
spectroscope. The light from the planet is passed
through the spectroscope, which contains one or
more prisms ; the result of this is to spread the light
out into a band or spectrum, showing the colours
of the rainbow, and each point in the spectrum
corresponds to a definite wave-length.

If sunlight is analysed by the spectroscope, the
light being admitted to the spectroscope through a
narrow slit, it is found that the spectrum is crossed
by a large number of fine dark lines, amounting to
many thousands ; to each of these lines there corre-
sponds a definite wave-length and a definite in-
tensity. They are known as the Fraunhofer lines,
after the physicist who first investigated them. The
analysis of this spectrum provides a great deal of
information about the composition of the Sun. If, for
instance, we pass an electric spark between two pieces
of iron and examine the spectrum of the incan-
descent vapour between them, we find a considerable
number of bright lines, spaced at irregular intervals
and of different intensities; this particular series
of lines is produced only by iron and by no other
element. It is characteristic of iron ; in a similar way,
every other element has its own characteristic spec-
trum. If, now, light from a hot incandescent source,
which has a continuous spectrum showing all the
colours of the rainbow, is passed through the vapour
of iron at a lower temperature, the continuous spec-
trum of the hot source is found to be crossed by a

METHODS OF INVESTIGATION 63

number of dark lines, each of which is exactly
identical in wave-length with one of the bright lines
in the spectrum of the incandescent iron vapour.
Such a spectrum is called an absorption spectrum.
The Fraunhofer lines in the spectrum of the Sun
are of this nature; the hot interior of the Sun would
give a continuous spectrum, but the cooler outer
layers absorb the radiations of various wave-lengths,
thus producing the dark Fraunhofer lines.

When we investigate the spectrum of sunlight, we
find that the spectrum of iron, line by line, is
contained in it, proving conclusively that there is
iron in the Sun. In a similar way, we can detect
the presence of one element after another in the
Sun and learn much about the elements contained
in the Sun ; we can, moreover, go farther than this
and use the relative intensities of the lines due to
different elements to obtain some fairly reliable
conclusions about the abundance of each element.
If, for instance, we were to double the amount of
one element in the Sun, leaving the amounts of the
other elements unaltered, we should find the
intensities of the lines of that particular element
would be relatively strengthened. It is by means
of such considerations that we can determine the
relative abundance of this or of that element in the
Sun, or in a remote star or nebula.

There is, however, one complicating factor that
enters into the analysis of the light from the Sun.
We make our observations from the bottom of our
extensive atmosphere. The light from the Sun has
to pass through this atmosphere before it reaches us
and some of the light is absorbed in the atmosphere.

64 LIFE ON OTHER WORLDS

The consequence is that some of the absorption
lines that are present in the observed spectrum of the
Sun do not originate in the Sun but in the atmo-
sphere of our Earth. Of particular importance to
us is the absorption produced by the ozone in the
atmosphere. The amount of ozone in the atmo-
sphere is extremely small; it is estimated to be
equivalent to a layer about one-tenth of an inch
thick at atmospheric pressure and room tempera-
ture. It occurs almost exclusively above the
highest clouds, the greatest density being at a
height of between twenty and thirty miles. Small
though the amount of ozone is, the absorption pro-
duced by it in the ultra-violet region of the spectrum
is so strong that all of the light of wave-length shorter
than O'oooois inch is completely absorbed; none
of the light in this region of the spectrum is con-
sequently accessible to observation. 1 Unfortunate
though this is for the investigations of the astron-
omer, it is a fortunate circumstance for life as we
know it: for animals, including human beings,
could not exist as they are now constituted, if there
were not a small amount of ozone in the atmosphere.
The importance of ozone biologically is this:

1 The approximate limits of wave-length for the different colours
of the spectrum are as follows :

Ultra-violet L 0-000014 inch.

Violet 0-000014 to 1 8 inch.

Blue 0-000018 to 20 inch.

Green 0*000020 to 22 inch.

Yellow 0-000022 to 24 inch.

Orange 0-000024 to 26 inch.

Red 0-000026 to 30 inch.

Infra-red 7 0-000030 inch.

METHODS OF INVESTIGATION 65

much of the radiation of short wave-length that is
absorbed by ozone is very injurious to the eye and
has an injurious effect also on the other tissues of the
body. The light from glowing mercury vapour is
rich in the ultra-violet light. Quartz is fairly
transparent to such light, whilst glass absorbs it
strongly. A quartz mercury vapour lamp is
therefore a powerful source of ultra-violet light and
such lamps are accordingly used for ultra-violet
light treatment; anyone who has undergone such
treatment knows that the exposure to the lamp must
be carefully timed, otherwise severe damage to the
tissues may result, and that it is necessary to wear
dark spectacles for the protection of the eyes during
the exposure to the rays. In moderate amount,
however, the ultra-violet light is beneficial because
it keeps us in proper health and is effective in pre-
venting rickets. If, then, there were no ozone in
the atmosphere, eyes as now constituted could not
have developed and the bodily tissues would be
seriously injured; if, on the other hand, the ozone
were present in greater amount, life, as we are
familiar with it, could not continue. It is not to be
concluded that no animal life would be possible if
there were no ozone in the atmosphere; this is
merely a striking illustration of how life has
adapted itself to the conditions that prevail. In
this particular instance, however, the adaptation
is surprisingly close. The amount of ozone in the
atmosphere is not constant, it is variable within
somewhat narrow limits but these limits fortunately
lie between the two extremes, the one at which
on the one hand the amount of ultra-violet light

66 LIFE ON OTHER WORLDS

would become destructive of life and the other at
which the amount would become insufficient to
maintain it.

The oxygen in the atmosphere produces some
strong absorptions in the spectrum of the Sun;
these are mainly in the regions of longer wave-
length, the red and the near infra-red regions of the
spectrum, though there are also some weaker
absorptions of shorter wave-length in the visual
spectrum. The two strong absorptions in the red
region of the spectrum that were called the A and
B bands by Fraunhofer are caused by oxygen.
Water-vapour in the atmosphere produces some
extremely strong absorptions in the long wave-
length infra-red region. Nitrogen, on the other
hand, though it is the most abundant constituent
of the atmosphere, produces no absorptions in
the region with which our investigations are
concerned.

When we analyse the spectrum of the Sun, how
are we to distinguish between the absorptions that
originate in the Sun itself and those that are pro-
duced by our atmosphere ? There are two ways
in which the absorptions of terrestrial origin can be
identified. The first method is to compare the
spectra of the Sun taken at different altitudes. The
lower the altitude of the Sun, the longer is the path
that the light from the Sun has to travel through the
atmosphere in order to reach us; it is, in fact, for
this very reason that the light from the Sun^ is
much less intense towards sunset than it is at mid-
day. The longer the path through the atmosphere
the greater become the effects of atmospheric

METHODS OF INVESTIGATION 67

absorption. The lower the altitude of the Sun,
therefore, the more intense are the absorptions of
terrestrial origin relative to those of solar origin.
The second method is to compare the spectra of
light from the east and west limbs of the Sun. The
Sun rotates on its axis in about twenty-seven days;
as a result of this rotation the west limb is moving
away from us whilst the east limb is moving towards
us. The wave-length of the light received from a
source that is moving towards us is shortened and
the absorptions are therefore displaced slightly
towards the violet end of the spectrum ; the wave-
length of the light from a source that is moving
away from us is lengthened and the absorptions are
therefore displaced slightly towards the red end of
the spectrum. Comparing the spectra of the east
and west limbs of the Sun, we consequently find
that there is a slight separation between the corre-
sponding absorptions in the light from the two
regions. The absorptions that originate in our
atmosphere are not affected by the rotation of the
Sun and these absorptions coincide in position in
the spectra of the light from the two opposite limbs.
If, then, the spectra of the eastern and western
limbs of the Sun are photographed simultaneously
or in immediate succession on the same plate, the
absorptions of terrestrial origin can be at once
picked out, because they consist of all the absorptions
that coincide in position in the two spectra. This
method has the advantage over the first method that
it does not necessitate the comparison of spectra
obtained at different altitudes and therefore at
different times.

68 LIFE ON OTHER WORLDS

Having by one or other of these methods identi-
fied the absorptions that originate in the atmosphere
of the Earth, the way is open to learn something
about the composition of the atmospheres of the
planets. Let us first consider for a moment the
nature of the light that we receive from a planet.
The planets are cool bodies and have no intrinsic
light of their own. We see a planet by means of
light from the Sun that falls upon it and is reflected
back. As the sunlight penetrates into the atmo-
sphere of the planet, it is partially scattered and
partially absorbed. The depth to which it pene-
trates depends upon the nature and extent of the
atmosphere; the light may or may not actually
penetrate to the surface of the planet. Some of the
light that reaches us from the planet will have
penetrated to a greater depth into the atmosphere
of the planet and some will have penetrated into
a lesser depth; the net effect may be expected to be
that the light will bear some impress of its passage
into and out of the atmosphere of the planet, so
that when the light is analysed by the spectroscope,
absorptions that have originated in the atmosphere
of the planet may be revealed, which will serve to
give some clues to the nature of the atmosphere.
An absorption in the atmosphere of the planet
that does not correspond with any absorptions pro-
duced in the atmosphere of the Earth will be
readily revealed. On the other hand, it may
happen that the atmosphere of the planet contains
oxygen or water-vapour, whose absorptions will
coincide with the absorptions produced by the
same substances in the Earth's atmosphere. Some

METHODS OF INVESTIGATION 69

care is required, therefore, to decide whether the
absorptions produced by oxygen and water-vapour
arise entirely in the atmosphere of the Earth or
whether they include the effect of absorptions
originating in the atmosphere of the planet.
Oxygen and water-vapour are the two substances
whose presence in the planetary atmosphere is of
the greatest significance for the possibility of the
existence of life and it is these two substances whose
presence may be most difficult to establish.

Two methods can be used to distinguish between
an absorption of terrestrial origin and the same
absorption of planetary origin. One method is to
compare the spectrum of the planet with the spec-
trum of the Moon, the two spectra being obtained
on the same night and as nearly as possible at the
same time. The Moon and the planet should be at
equal altitudes when their spectra are photographed,
so that the absorptions produced by the Earth's
atmosphere will be equal, or very nearly equal, in
the two cases, because the air-paths are equal.
The Moon, as we shall see, is devoid of atmosphere;
it follows that if an absorption is present in the
spectrum of the planet and not in that of the Moon,
or is stronger in the spectrum of the planet than in
that of the Moon, it must originate in the atmo-
sphere of the planet. If the absorption produced
by the atmosphere of the Earth is very much more
intense than that produced by the atmosphere of
the planet, we may fail to detect in this way the
planetary absorption, because the difference in the
intensities of the absorptions in the two spectra
may be too small. The second method of invest!-

7O LIFE ON OTHER WORLDS

gation is a more delicate one, and is specially useful
for deciding whether substances that are present
in our own atmosphere are also present in the
atmosphere of the planet. It is based on the dis-
placement of the absorption lines in the light from
a moving source, which we have already used for
the detection of the absorptions produced by the
Earth's atmosphere from the comparison of the
light from the east and west limbs of the Sun.
The spectrum of the planet is photographed at a
time when the planet is approaching or receding
from the Earth most rapidly. The relative motion
will displace the absorptions due to the planet's
atmosphere with respect to those due to our own
atmosphere and in this way we may hope to detect
planetary absorptions of low intensity.

It must be emphasised that though by these
methods we may expect to obtain some informa-
tion about the constitution of the atmosphere of the
planets, the information can never be complete.
There are many possible constituents of an atmo-
sphere that show no absorptions in the region of the
spectrum that is accessible to study. There is no
means available by which such constituents can be
detected. Amongst these undetectable constituents
are hydrogen, nitrogen, helium, neon and argon.

A knowledge of the temperatures of the planets
is of importance for our consideration of the
possibility of the existence of life. Some general
information about the temperature conditions can
be obtained from theoretical considerations.

We shall assume, to begin with, that the planet
has no atmosphere, because the effects of an

METHODS OF INVESTIGATION 71

atmosphere on the temperature of the planet are
complicated and difficult to estimate. We shall
consider first the extreme case of a planet which,
like Mercury, always turns the same face to the
Sun. We can estimate the temperature of the
surface of the planet if we suppose that the planet
has no output of heat of its own, but that there is an
exact balance between the heat that it receives
from the Sun and the heat that it radiates into
space. From the measurements of the radiation
from the Sun received at the surface of the Earth,
we know that the energy from the Sun reaching
the Earth just outside the Earth's atmosphere is
equivalent to 1-54 horse-power each square yard.
If the distance of the planet from the Sun is R times
the distance of the Earth from the Sun, the point on
the surface of the planet where the Sun is overhead
receives energy at the rate of i '54/R 2 horse-power
per square yard. This must be equivalent to the
rate at which heat is radiated. This rate is deter-
mined by the temperature of the planet. In 1879
Stefan put forward the suggestion that the total
radiation from a body is proportioned to the fourth
power of its absolute temperature (i.e. the tempera-
ture measured from the absolute zero, 273 C.).
This law is accurately true only for what is termed
a " black body," a body that completely absorbs all
the radiations falling on it. The radiation from
the planets is not strictly in accordance with
Stefan's law, though we can use the law to derive
an estimate of the temperature that will be suffi-
ciently accurate for our purpose. The constant of
proportionality, which enters into Stefan's law,

72 LIFE ON OTHER WORLDS

has been found by experiment. Using this observed
value and equating the energy received and radiated,
it is found that the temperature (on the absolute
scale, denoted^ by K) is given by the expression
T = 392 / A/R. At the distance of the Earth, where
R = i, the temperature obtained is 392 K, or
119 C. The temperature found in this way is the
highest temperature at any point on the surface of
a planet that always turns the same face to the
Sun provided the assumptions that the planet has
no atmosphere and that it is a "black body 5 ' are
satisfied. At other points on the planet the tem-
perature would be lower, because the Sun's rays
would fall obliquely instead of vertically; the dark
side of the planet, which receives no heat from the
Sun, would be extremely cold.

If the planet does not always turn the same face
to the Sun, the effect of its rotation would be to
lower the noonday temperature and to raise the
night temperature; the noonday temperature would
be lowered because no point of the surface would
have the Sun in its zenith for more than a short
time and the night temperature would be higher
because the cooling off of the surface that had been
heated by the Sun in the daytime would be gradual.
The faster the rotation, the smaller would be the
difference between the day and night temperatures.
For a sufficiently fast rotation, there would be no
difference between day and night temperatures
at any place on the planet; there would be, how-
ever, a variation of temperature with latitude,
because the average rate of reception of heat from
the Sun depends on the latitude. The average

METHODS OF INVESTIGATION 73

temperature over the whole surface under these
conditions can be estimated.

Suppose the planet to be at the distance of the
Earth and its radius to be r yards. The planet is
receiving energy from the Sun at the rate of 1-54 x
?rr 2 horse-power, because the cross-sectional area
exposed to the radiation is Ttr 2 . If the distance
of the planet from the Sun is R times the distance
of the Earth from the Sun, the corresponding rate
is 1-54 X 7rr 2 /R 2 horse-power. The total radiation
of energy from the whole surface (4^r 2 ) is given,
in terms of the temperature T, by Stefan's law.
Equating the energy received with the energy
radiated, we obtain T = 277/VR, expressed on
the absolute scale. It may be noted that the average
absolute temperature under these conditions will
be equal to the maximum temperature previously
found, divided by V2. The average temperature
for the Earth under these conditions would be 277
K or 4 C.

The temperature on a planet that possesses an
atmosphere cannot be so readily calculated, be-
cause complex meteorological effects come into
play and, as is well known on the Earth, tempera-
tures at any one place may differ considerably
from day to day. The general effect of an atmo-
sphere is nevertheless readily seen to be a smoothing
out of the temperature differences between day
and night, because there will be a persistent
tendency for heat to be carried from the warmer to
the cooler parts of the surface by warm air moving
into colder regions and cold air moving into
warmer regions.

74 LIFE ON OTHER WORLDS

The temperature at the surface of a planet is
influenced by its atmosphere in another way.
The atmosphere of the Earth, and probably also
most planetary atmospheres, are opaque in many
regions of the infra-red, corresponding to long
wave-length radiations. Most of the solar heat
is transmitted by the atmosphere, warming the
surface of the planet; much of this heat is radi-
ated again as radiation of considerably longer
wave-length, to which the atmosphere is opaque.
The temperature is therefore raised considerably.
By preventing the escape of the radiations of long
wave-length, the atmosphere exerts a blanketing
effect and the fall of temperature at night becomes
less rapid; the diurnal range of temperature is
therefore appreciably reduced.

The table on p. 76 gives a comparison between
the measured temperatures and the temperatures
estimated on the two hypotheses mentioned above.
The second column gives the directly measured
temperatures; the third column gives the average
temperatures on the assumption that there is no
diurnal variation of temperature; the fourth
column gives the maximum temperature on the
sunlit face, assuming that the same face is always
turned to the Sun. The planets are arranged in
the table in the order of increasing distance from
the Sun, and therefore also of decreasing tempera-
ture. If the distance of the Earth from the Sun
is represented by one foot then, on the same
scale, the distances of the other planets are
approximately as follows: Mercury, 4^ inches;
Venus, 8 inches; Mars, 18 inches; Jupiter,

METHODS OF INVESTIGATION 75

5 feet; Saturn, gf feet; Uranus, 19 feet; Neptune,
30 feet; Pluto, 40 feet.

The radiation received on the Earth from the
planets can be measured with the aid of a large
telescope, to gather as much of the radiation as
possible, in conjunction with a sensitive detector
of radiation. For detecting and measuring the
radiation a bolometer or a thermocouple may be
used. In the bolometer the radiant energy is
focused on to a minute strip of platinum, which
forms one arm of an electrical circuit known as a
Wheatstone's bridge. A similar strip, shielded
from the radiation, forms a second arm of the
bridge, which is balanced against the first. When
the radiant energy falls on the first arm of the bridge,
it is heated, its resistance is increased, the balance
of the bridge is upset and a current flows through
the galvanometer of the bridge. The deflection of
the galvanometer provides a measure of the
intensity of the radiation falling on the bolometer.
The thermocouple consists of a small junction of
two^ minute strips of different metals; when the
junction is heated a thermoelectric current, whose
strength is proportional to the intensity of the
radiation, flows through the circuit and is measured
by a sensitive galvanometer. A highly sensitive
thermocouple will detect the heat from a candle at
a distance of three miles.

The thermocouple or bolometer provides a
measure of the total radiation from the planet, as
modified by absorption in the Earth's atmosphere.
A correction must be applied to the measures to
allow for this effect, but we need not enter into the

76 LIFE ON OTHER WORLDS

details of this correction. When it is applied we
obtain a measure of the total planetary radiation,
which consists in part of reflected sunlight and in
part of the low-temperature long-wave radiation
from the planet itself. It is the latter portion that
provides information about the planet's tempera-
ture and it is necessary, therefore, to separate it from
the portion that is merely reflected sunlight. The
separation is easily effected by placing a small
transparent vessel containing water in the path of
the rays. The water transmits the portion of the
radiation of relatively short wave-length the
reflected sunlight portion but is opaque to the
long- wave planetary portion. We are thus enabled
to measure the true heat radiation from the planet.
The measured temperatures are given in the second
column of the table.

PLANETARY TEMPERATURES

Measured

Mercury (mean, sunlit side)
Venus (bright side)
Venus (dark side)
Earth . . .

Moon (centre of sunlit side)
Moon (centre of dark side)
Mars (hottest portions) .
Jupiter (average)
Saturn (average) .
Uranus (average) .
Neptune
Pluto ....

400
55

20

I20J

-ISO/
20

- I 4

- 155

< 180

Calculated
I II

172

54

4
4

183

210
222
229

358

43

100

184

2OI
211

METHODS OF INVESTIGATION 77

It will be noted that in general the measured
temperatures lie between the temperatures given
in columns three and four, and that there is a close
general agreement between the estimated tem-
peratures and those that are actually observed.
The measured temperatures will be commented
upon in subsequent chapters when dealing with the
individual planets.

CHAPTER IV

THE EVOLUTION OF THE
ATMOSPHERE OF THE EARTH

IN the last chapter we found that the loss of atmo-
sphere from a planet will be very rapid if the
velocity of escape from the planet is less than four
times the average velocity of the molecules and that
the atmosphere will be practically immune from
loss if the velocity of escape is more than five times
the average velocity of the molecules. Let us apply
these conclusions to the Earth and see whether the
atmosphere of the Earth is in accordance with
expectations.

The velocity of escape from the Earth is 7-1 miles
a second. Any component of the atmosphere,
whose average molecular velocity is less than about
i -4 miles a second, should therefore be practically
immune from loss. The average molecular velo-
cities for different gases are given on p. 56; it
will be seen that they are all less than the critical
value of 1*4 miles a second. These velocities
correspond to a temperature of oG.; for higher
temperatures the velocities will be greater. The
temperatures that occur on the Earth at the present
time are not great enough, however, to bring the
average molecular velocity of hydrogen to a value
as great as i -4 miles a second ; this velocity requires
a temperature of 88 C.

It appears, then, that the atmosphere of the
Earth should be immune from the loss of hydrogen

78

EVOLUTION OF THE ATMOSPHERE 79

at the present time and, therefore, immune also
from the loss of all other gases. The Earth should,
therefore, have retained the whole of its initial
atmosphere, unless the rate of loss of the atmo-
sphere soon aftqjr the Earth was formed, during the
period when the Earth was very much hotter than
it now is, was so rapid that in the course of a few
thousand years much of the atmosphere was lost.
There is evidence, which we shall consider presently,
that such a loss did indeed occur.

The estimated composition of the atmosphere,
by volume and by weight, according to the figures
given by Humphreys, is as follows:

COMPOSITION OF EARTH'S ATMOSPHERE

Constituent

Volume, per cent, of
dry air, at surface.

Weight in units of
100 million tons.

Total atmosphere

_

50,293,000

Dry air

100-00

50,162,360

Nitrogen .

78-03

38,111,360

Oxygen

20-99

11,413,080

Argon

0-9323

609,040

Water-vapour

130,490

Carbon dioxide

0-03

2I,3l6

Hydrogen

O-OI

1,270

Neon

0-0018

678

Krypton

o-oooi

126

Helium

0-0005

79

Ozone

0-00006

29

Xenon

0-000009

17

In addition, there are varying quantities of
impurities such as sulphur compounds, ammonia,
nitric and nitrous acids, particles of salt from the

PLATE t

THE MOON: REGION OF COPERNICUS

Copernicus is the large crater at the centre of the
photograph. It has a diameter of 56 miles. The
mountain-ring, which reaches a height of 11,000 feet
above the floor of the crater, falls gradually on the
outside but very precipitously, with deep ravines, on the
inside. The white streaks or rays, which radiate from
Copernicus, are well shown ; it will be noticed that they
pass over mountains and across craters.

To the right of Copernicus are some hundreds of
small craters, about 400 to 500 feet in diameter, many
of which are arranged in rows.

The range of mountains in the upper left-hand por-
tion of the photograph is known as the Carpathians.

Photographed by Dr. J. H. Moore and Mr. J. F.
Chappell, with the 36-inch refractor, Lick Observatory,
1937, October 26.

PLATE 7

THE PLANET JUPITER

The upper portion of the plate shows two drawings
of Jupiter by Rev. T. E. R. Phillips. The left-hand
drawing was made on 1908, April 24, and shows
Satellite III (Ganymede) as a dark spot in transit across
Jupiter. The drawing shows the common appearance
of the Red Spot, seen as a prominent oval near the
satellite, during conjunction with the South Tropical
Disturbance. The right-hand drawing was made on
J 933> March 9. It shows Satellite I (lo) partly occult-
ing its shadow, near the left-hand limb of the planet's
disk. Satellite IV (Gallisto) is shown in transit on the
lower part of the disk and casting its shadow on the disk.

These two drawings show nearly the same presenta-
tion of the planet and illustrate the changes in the
markings, which are atmospheric markings and not
surface details.

In the lower portion of the plate tv/o photographs of
Jupiter taken by Dr. Jeffers with the 36-inch refractor
of the Lick Observatory on 1939, October 13 (left) and
October 21 (right). These photographs show clearly
the complex system of dark belts on Jupiter. In the
right-hand photograph the Red Spot may be seen above
and to the left of the centre.

82 LIFE ON OTHER WORLDS

evaporation of sea spray, particles of soot, fine
dust, and pollen of many varieties.

The existence of the rare gas, argon, in the
atmosphere was discovered by Lord Rayleigh and
Sir William Ramsay in the year 1894. It may
appear surprising that, although one per cent, by
volume of the air we breathe is argon, this gas
was not discovered at a much earlier date. The
reason is that argon is a very inert gas and does not
form chemical compounds, so that its existence was
not recognised. It will be seen from the above table
that the weight of argon in the atmosphere is several
times the total weight of the water-vapour, carbon
dioxide, and every other atmospheric constituent,
except the two principal constituents, nitrogen and
oxygen. In the years 1895-98, shortly after the
discovery of argon, Sir William Ramsay and his
assistant, Travers, discovered the four other rare
gases, helium, neon, krypton and xenon.

Helium, as previously mentioned, is the only
substance that was discovered in the Sun before it
was found on the Earth. First discovered on the
Earth by Ramsay as a constituent of the atmosphere,
it was later found to be present in cleveite and other
uranium- and thorium-bearing minerals; helium
gas, given off by such minerals, is collected from
certain bore-holes in the United States. Helium
being the lightest gas with the exception of hydro-
gen, and not forming an inflammable mixture with
air, has been used for filling the balloons of air-
ships.

Argon is obtained today in a state of compara-
tive purity from commercial oxygen-distilling

EVOLUTION OF THE ATMOSPHERE 83

apparatus and some ten million cubic feet are used
each year for filling electric-light bulbs, the so-
called gas-filled bulbs, because the efficiency of
the lamp is thereby increased and its life is pro-
longed. Neon is extensively used for advertising
signs. Krypton and xenon have not been used
much for commercial purposes but they may
eventually be used in electric-light bulbs, as they
increase the efficiency of the lamp still further.

The amount of water-vapour in the atmosphere
is very variable, depending upon temperature and
other conditions. Near the Earth's surface, the
proportion of water-vapour by volume may vary
from a mere trace to about five per cent, on hot days
of very high humidity. The water-vapour is
found only in the lowest layers of the atmosphere;
there is very little above a height of five miles, be-
cause of the low temperature at the higher levels.
If all the water-vapour in the Earth's atmosphere
were condensed, the condensed water would be
sufficient in amount to cover the whole of the
surface of the Earth to a depth of about one
inch.

The composition of the atmosphere in its lower
layers, up to a height of about ten miles, is practi-
cally constant, except for the water-vapour con-
tent; in this region there is continual mixing of the
constituents by convection and the action of
turbulence. In the upper regions of the atmosphere
there is very little vertical movement and the distri-
bution of the constituents is therefore controlled
by the action of gravity. The percentage of the
lighter gases must, therefore, increase in the upper

84 LIFE ON OTHER WORLDS

regions, and at the highest levels hydrogen and
helium must be the principal constituents.

It is not possible to state how high the atmo-
sphere extends above the surface of the Earth.
The density of the atmosphere decreases with
height, gradually thinning out until empty space
is reached; but there is no sudden transition from
air to empty space. Shooting stars become
visible at heights of from 70 to 100 miles; these
small bodies, which enter the Earth's atmosphere
from outside, are not seen until the friction caused
by their rapid motion through the air heats them
to such an extent that they become incandescent.
The aurora borealis, or northern lights, gives evi-
dence of the extension of the atmosphere to much
greater heights. The auroral light is an electrical
phenomenon, caused by the entrance of electrified
particles into the atmosphere. By taking photo-
graphs of the aurora from two places, some miles
apart, the height can be found. The lower edge
of the aurora is usually at a height of some 60 or 70
miles; the highest portions have been found to
extend to heights as great as 500 to 600 miles.
The density at such heights is so extremely small
that we may regard the effective limit of the atmo-
sphere as being about 600 miles above the surface.
If the atmosphere were compressed so that it was
of uniform density throughout, the density being
equal to the actual density at the surface, it would
extend to a height of only 5^ miles; this is called the
height of the equivalent atmosphere.

It is of some interest to note that the total weight
of the Earth's atmosphere is rather less than a

EVOLUTION OF THE ATMOSPHERE 85

one-millionth part of that of the Earth itself and
that the weight of the atmosphere is equivalent
to that of an ocean of water covering the whole of
the Earth's surface to a depth of about thirty-three
feet. The least abundant of the constituents,
xenon, if loaded on railway wagons, each carrying
ten tons, would require a train to carry it that would
extend eighty times around the Earth's equator.
Travelling at twenty miles an hour, such a train
would take twelve years to pass by.

We have seen that helium is present in the
atmosphere to the extent of about five parts in one
million by volume. Helium is being continually
added to the atmosphere by the process of the
weathering of the igneous rocks of the Earth's crust,
which contain uranium and thorium. In any
mineral or rock that contains these elements,
radioactive disintegrations are continually taking
place, one result of the break-up of these heavy
atoms being the formation of helium. Some of the
helium so produced remains inside the minerals
and rocks and some escapes into the atmosphere;
the proportion that escapes depends on various
geological factors. When such rocks are decom-
posed by weathering, the whole of their helium
escapes into the atmosphere. It has been estimated
that the atmosphere does not now contain more
than a fraction of the amount of the helium that it
has gained during geological times in the process
of the formation of the sedimentary rocks as a
result of the weathering of the igneous rocks. It
must be concluded that much of the helium that
has been added to the atmosphere in this way has

86 LIFE ON OTHER WORLDS

somehow escaped; the suggestion has been made
that there may be at the present time an approxi-
mate balance between the amount of helium that
is still being added in the way we have described
and the amount that is being lost.

It may be objected that the loss is, possibly, only
apparent and that the helium, being a light gas,
has become concentrated in the upper regions of
the atmosphere where little or no mixing up of the
constituents can occur by convection. There is
direct evidence, however, that the upper atmo-
sphere cannot be rich in helium because the
spectrum of light from the aurora, which comes
from a height of something like sixty miles, shows
the presence of oxygen and nitrogen but not that of
helium.

Even if the Earth had remained sufficiently hot
in the early stages of its existence for a time long
enough for the hydrogen and helium then present
in its atmosphere to escape entirely, it still remains
to explain how helium continues to be lost when,
according to the theoretical conclusions already
mentioned, which are based on the accepted prin-
ciples of the kinetic theory of gases, it should not be
possible for it to escape. There is one process by
which the escape of helium can be brought about.
It is well known that the night sky is faintly lumin-
ous. In addition to the light from the stars there is
a faint luminescence from the upper atmosphere,
whose brightness seems to vary with the sun-spot
cycle, being greater at sun-spot maximum than at
sun-spot minimum. Lord Rayleigh has termed
this luminescence the non-polar aurora. In the

EVOLUTION OF THE ATMOSPHERE 87

spectrum of this faint light from the night sky,
obtained by taking long exposures with special
spectographs of small dispersion, the characteristic
green and red lines that occur in the spectrum of
the bright aurora borealis are always present.
These particular lines are known to be produced by
radiations from atoms of oxygen that are in a
special condition, which the physicists term a
metastable state. An atom, when excited or loaded
up with energy, usually unloads its energy within
a short interval of time of the order of one hundred-
millionth of a second ; this unloading of the energy
corresponds to the emission of radiation. A meta-
stable state, on the other hand, is characterised by
the peculiarity that the atoms in that state do not
have a very strong inclination to unload their
energy : they may remain for an average time of a
second or longer before emitting their energy in
the form of radiation. But collisions between the
atoms are so frequent that there is a high proba-
bility that before an atom in the metastable state
emits its energy as radiation it will have collided
with another atom. Whenever a collision of a
metastable oxygen atom with another atom occurs,
the energy of the oxygen atom which in due
course would have been emitted as radiation is
immediately unloaded and is converted into kinetic
energy. Instead of being emitted as radiation, it is
used in making the two colliding atoms rebound
with a greatly increased speed. The amount of
energy that is unloaded in this way when another
atom collides with a metastable oxygen atom can
be computed. It is found that if the colliding

88 LIFE ON OTHER WORLDS

atom is an atom of helium the energy is sufficient to
enable the atom of helium to rebound with a speed
of more than seven and a half miles a second. As
this speed is greater than the velocity of escape from
the Earth, the atom of helium has a chance of
escaping, which it would not otherwise have.
Hydrogen atoms would acquire a still greater speed
and could also escape. Heavier atoms, such as
those of nitrogen and oxygen, though they would
receive an equal amount of energy by the collisions,
would not acquire such large velocities ; they would
not rebound with a speed greater than the velocity
of escape. The loss of hydrogen and helium from
the atmosphere of the Earth at the present time is
thus made possible by this special process, depend-
ing upon the fact that free oxygen is present in the
atmosphere. It is possible in this way to explain
satisfactorily why the amount of helium in the
atmosphere at the present time is less than we should
have expected.

There is strong evidence that the present atmo-
sphere of the Earth is not its original atmosphere
and that the primitive Earth must have remained
hot sufficiently long for most of the initial atmo-
sphere to have been lost. This evidence is provided
by the comparison between the relative abundance
of different elements on the Earth and the relative
abundance of the same elements in the Sun and
the stars. We must here anticipate a subject that
will be discussed more fully in a later chapter. It
is believed that the Earth, in common with the other
planets in the solar system, was formed from matter
drawn out from the Sun by the gravitational action

EVOLUTION OF THE ATMOSPHERE 89

of another star that passed close by the Sun. If
this was so, and there is no satisfactory alternative
theory to account for the origin of the solar system,
we should expect that the composition of the Earth
would be generally similar to that of the outer layers
of the Sun.

The investigation of the chemical composition of
the atmosphere of the Sun is based upon the study
of its spectrum; the number of atoms needed to
produce a line of given intensity in the spectrum of
the Sun can be estimated. The composition of the
Earth is estimated from the actual analysis of
typical rock samples, the results being combined in
proportion to the relative amounts of the different
kinds of rocks, as inferred from geological evidence.
This information is supplemented by information
about the interior of the Earth provided by the
study of seismograph records of earthquakes, which
provide information about the propagation of the
earthquake waves through the interior of the Earth,
and hence about the nature of the interior. There
are, naturally, uncertainties attaching to both
methods, because in neither case is the available
data as complete as we should like. The com-
parison of the conclusions from the two entirely
different methods of investigation is, nevertheless,
of great interest. The most reliable information
about the composition of the atmosphere of the
Sun is obtained for the metallic elements. Russell
gives the following table, in which the fourteen
most abundant metals in the Sun, found from
his investigation of the Sun's spectrum, are com-
pared with the fourteen most abundant metals

9O LIFE ON OTHER WORLDS

in the Earth, as indicated by the work of Gold-
sehmidt:

COMPARISON OF ABUNDANCE OF ELEMENTS IN EARTH AND SUN

Group. Earth. Sun.

I . Iron Magnesium

Magnesium Sodium

Aluminium Iron

Nickel Potassium

Calcium Calcium

Sodium Aluminium
Potassium

II . . . Titanium Manganese

Chromium Nickel

Manganese Chromium

Cobalt Cobalt

Titanium

III . . . Copper Vanadium

Vanadium Copper

Zinc Zinc

In the above table, the elements are divided into
three groups. The average abundance of elements
in the first group is about ten times that of elements
in the second, and the average abundance of ele-
ments in the second group is about ten times that of
elements in the third.

The same fourteen metals appear in the two
columns headed Earth and Sun and the grouping
into the three groups is generally similar.

The analysis of meteorites gives the following
as the most abundant metallic elements: iron,
magnesium, sodium, nickel, calcium, aluminium.
The meteorites are probably fragments of matter
drawn from the Sun when the planets were formed,
which did not condense into planets. It will be

EVOLUTION OF THE ATMOSPHERE gi

noticed that the six most abundant metals in the
meteorites are included in Group I in the list of
most abundant metals in the Earth.

The investigation has been extended to include
the metals of lower abundance than those given in
the above table; when the lists of relative abundance
for the Earth and the Sun are compared, it is found
that out of a total of forty-eight metals there are
only four cases in which one list makes a given
element more than ten times as abundant as the
other, though some of these metals are a hundred
thousand times more plentiful than others. The
four cases of greatest discordance all relate to metals
for which either the spectroscopic data for the Sun
or the chemical data for the Earth are known to be
rather uncertain. The conclusion drawn by Rus-
sell from this detailed investigation is that " it is
hard, indeed, to find a single case in which we can
be sure that a given metal is more or less abundant
in the Sun than on the Earth."

This striking similarity in composition may be
regarded as providing substantial evidence of a
common origin. It is just what we might have
anticipated if the Earth was formed from material
that had been drawn out from the Sun. It is
rather more surprising to find that a detailed in-
vestigation of the spectra of the stars, made in a
manner analogous to that used for the analysis of
the spectrum of the Sun, shows that the composition
of the stars is substantially similar to that of the Sun.
This seems to suggest that the Sun and all the stars
have themselves been formed from some primaeval
material whose composition was everywhere more

92 LIFE ON OTHER WORLDS

or less the same and that subsequent processes of
building up more complex atoms out of simpler
ones, or of the breaking up of the most complex
atoms, have followed a generally similar course.

When we turn from the metals to the non-metals
we find an entirely different picture. The striking
similarity between the composition of the Earth
and that of the Sun and the stars no longer persists.
Some remarkable differences are shown. Hydro-
gen, for instance, is far more abundant in the Sun
than in the Earth. At least 90 per cent, of all the
atoms in the atmosphere of the Sun, and perhaps
95 per cent, or more, are atoms of hydrogen ;
there are some three hundred times as many atoms
of hydrogen in the Sun as there are atoms of all
the metals together. The number of atoms of
hydrogen in terrestrial rocks is about equal to the
average number of atoms of the six most abundant
metals contained in the rocks : aluminium, iron,
calcium, sodium, potassium and magnesium. It
might be thought that the oceans would account
for much of the terrestrial hydrogen, as each mole-
cule of water contains two atoms of hydrogen ; but
the oceans only contribute 239 parts by weight out
of one million parts for the Earth as a whole.

Another striking discordance is provided by
nitrogen. Nitrogen is very abundant in the Sun,
the stars and the nebulae. The nitrogen in the
Earth's atmosphere amounts to about one part by
weight in two million for the Earth as a whole;
there is a little nitrogen in the igneous rocks and
some may be dissolved in the liquid core of the
Earth. But with ample allowance for such possi-

EVOLU^MJN OF THE ATMOSPHERE 93

bilities, nitrogen^must be at least some hundreds
of times more abundant in the Sun and stars than
in the Earth.

Perhaps the most interesting discordances are
those shown by the rare gases of the atmosphere,
argon, neon, helium, krypton and xenon, because
these are the rarest of all the elements on the
Earth* Being very inert substances chemically,
they do not combine with other elements to form
compounds. With the exception of helium, which
is continually being formed by the breaking up of
the heavy radioactive elements, there is no reason
to believe that these elements are present in the
interior of the Earth; all that there is of them is
contained in the atmosphere and how little there is
of these elements in the atmosphere, argon alone
excepted, may be seen from the table on p. 79,
which gives the composition of the Earth's atmo-
sphere. Some of these inert gases are known to have
a high cosmic abundance. The amount of helium
in the Sun is difficult to estimate with certainty but
in the hotter stars, in whose spectra the lines due to
helium are strong, helium is undoubtedly very
abundant. It has been estimated that the abun-
dance of helium on the Sun is about one hundredth
of that of hydrogen , the terrestrial abundance is
probably not more than one ten-millionth as great.
Neon lines are strong in the spectra of the nebulae
and the very hot stars, and it seems that the cosmical
abundance of neon is some 500 million times greater
than its terrestrial abundance. The discordance is
not so marked for argon, whose cosmical abundance
appears to be about equal to that of neon, whereas

94 LIFE ON OTHER WORLDS

it is several hundred times more abundant on the
Earth than neon. There is no information about
the cosmical abundance of krypton and xenon.

Of the gaseous elements that are present in the
afmosphere of the Earth the least discordance
between the solar and terrestrial abundance is
shown by oxygen. Though it is not easy to estimate
accurately the amount of oxygen in the Sun's
atmosphere, the available evidence suggests that
oxygen may be about equally abundant on the
Earth and on the Sun. Most of this oxygen occurs,
however, not in the free state in the atmosphere
but in chemical combination.

The similarity in the relative abundances of the
metals in the Sun and Earth suggested a common
origin. This common origin being accepted, the
great differences in the relative abundances of the
inert atmosphere-forming elements nitrogen, ar-
gon, helium and neon force us to the conclusion
that these gases have been to a very great extent
lost to the Earth. It is reasonable to suppose that
when the Earth was first formed these elements
were as abundant in the Earth as they are in the
Sun and the stars. Being elements that do not
readily enter into chemical combination with other
elements, when the Earth began to cool and the
temperature had fallen sufficiently for chemical
compounds to form, these gases remained to a large
extent in the Earth's atmosphere. But the Earth
at that time was still very hot, so that it was losing
its atmosphere at a very rapid rate, and the lighter
gases were naturally being lost at a faster rate than
the heavier gases. When it had cooled to such an

EVOLUTION OF THE ATMOSPHERE 95

extent that the loss of its atmosphere practically
ceased, neon had been depleted to a much greater
extent than the heavier argon. The atomic weight
of neon is twenty, whereas that of argon is forty, as
compared with atomic weights of fourteen and
sixteen for nitrogen and oxygen respectively. In
this way we have a plausible explanation of why
argon is five hundred times more abundant than
neon in the atmosphere of the Earth, whereas in
the Sun and in the stars, neon is as abundant as
argon.

The conclusion we have reached is that when the
Earth had cooled sufficiently for the escape of its
atmosphere practically to cease, almost all the neon
that had been contained in the initial atmosphere
had escaped. Much of the heavier argon had
naturally escaped also, but the depletion of the
argon was much less than that of the neon. Most
of the original atmospheric oxygen, nitrogen and
water-vapour, and almost all of the original
helium and free hydrogen, must also have been lost,
because these substances are lighter than neon. At
this stage in its history, the Earth must have been
almost devoid of an atmosphere.

As the molten Earth cooled still further, great
quantities of water-vapour, carbon dioxide and
other gases must have been evolved from the liquid
magma, when at length it solidified to form the
crust. These gases, with the residual gases from
the initial atmosphere, formed the new atmosphere
which, as the Earth was then relatively cool, could
not escape. This atmosphere differed from the
present atmosphere of the Earth in that it con-

PLATE 8

THE PLANET SATURN

In the upper portion of the plate is shown a photo-
graph of Saturn obtained by Dr. Jeflfers with the 36-inch
refractor of the Lick Observatory on 1939, October 21.
The outer ring has an exterior diameter of 171,000 miles
and a width of about 10,000 miles; the inner ring has an
outer diameter of about 145,000 miles and a width of
about 16,000 miles. The width of the division between
the rings is about 3,000 miles. The feebly luminous
inner ring is not shown in the photograph. The shadow
of the ball of Saturn on the ring will be noticed. The
belts of Saturn are shown, and it will be seen that they
are much less well-defined than the belts of Jupiter.

The lower portion of the plate shows photographs of
Saturn taken in ultra-violet, violet, yellow, and red light.
The differences in appearance are due to the different
degrees of penetration of the light of different wave-
length into the atmosphere of Saturn. The photographs
were taken by Dr. W. H. Wright with the Grossley
Reflector of the Lick Observatory, on 1929, August
25-26.

The different appearance of the rings at the two dates
may be mentioned. In 1929 the rings were wide
open, in 1939 they were viewed more obliquely. The
appearance of the rings depends upon the elevation of
the Earth above their plane, which can reach 27 as
its maximum. When the Earth passes through their
plane, the rings become almost invisible and appear
like thin needles projecting from opposite sides of the
planet.

s

CO

PLATE 9

THE SPECTRA OF THE MAJOR PLANETS

The plate shows in succession, from top to bottom,
the spectra of the Moon, Jupiter, Saturn, Uranus and
Neptune. The scale of wave-lengths is indicated at the
top (the unit in which the wave-lengths are expressed
is the " tenth-metre " one-ten-thousand-millionth part
of a metre so that a wave-length of 5,000 is equivalent
to -00002 inch). The spectrum of the Moon is that of
the sunlight reflected by the Moon. The absorption
lines designated A, B by Fraunhofer are caused by
oxygen in the atmosphere of the Earth; a is caused by
moisture in the atmosphere of the Earth, C and F are
caused by hydrogen in the Sun, D by sodium, E by iron
and b by magnesium in the Sun. A, a, B, C are in the
red, D is in the yellow, *, b are in the green and Fis in the
blue. These absorptions are present also in the spectra
of Jupiter, Saturn, Uranus and Neptune. Other
absorptions will be noticed in the spectra of these planets
which do not appear in the spectrum of the Moon.
These are produced by the atmospheres of the planets.
In the spectrum of Jupiter, just to the left of C, is a weak
absorption caused by ammonia. It is faintly visible in
the spectrum of Saturn but is not seen in the ^pectra of
Uranus and Neptune. Other absorptions increase in
strength from the spectrum of Jupiter to that of Neptune;
they are all produced by marsh-gas (or methane). They
are so intense in the spectra of Uranus and Neptune that
the yellow and red regions of these spectra are almost
entirely cut out. It is for this reason that these planets
show their characteristic green colour. A thickness of
25 miles of marsh-gas, at atmospheric pressure, is
required to give absorptions as intense as those in the
spectrum of Neptune.

7

98 LIFE ON OTHER WORLDS

tained a great amount of carbon dioxide and a
great amount of water-vapour but not much
oxygen. In course of time, as the Earth cooled
still further, most of the water-vapour condensed
out of the atmosphere and formed the oceans.

It remains to explain how the change from the
atmosphere, as it then existed, to the present
atmosphere has been brought about. For more
than a century it has been recognised that the
presence of free oxygen in the atmosphere, which
we are apt to take for granted, demands explana-
tion. Oxygen is an element that is chemically
very active. Whereas the rare gases in the atmo-
sphere are chemically inert and do not form com-
pounds with other elements, oxygen does not like
to exist alone. It is always eager to join up with
other elements to form oxides. The rusting of iron
is an illustration, rust being merely iron oxide.
Combustion is nothing more than a process of
oxidation, as we have already explained, and can-
not occur in the absence of oxygen : if a glowing
piece of wood is placed in ajar containing oxygen,
it will at once burst into flame, because the oxida-
tion then proceeds at a greatly enhanced rate.

Because of this preference of oxygen for com-
bining with other elements rather than for existing
alone, it must follow that processes are in continual
operation that are depleting the store of oxygen in
the atmosphere. One of the principal sources of
depletion arises from the weathering of the igneous
or basic rocks in the Earth's crust, a process leading
to the formation of sedimentary deposits. The
weathered material is carried down by streams and

EVOLUTION OF THE ATMOSPHERE 99

rivers and ultimately deposited on the sea floor as
sand, clay or mud. The iron contained in the
igneous rocks is not completely oxidised; the
greyish hue of these rocks results from the iron being
present mainly in the form of ferrous oxide, an
oxide of iron in which the iron has not got the full
complement of oxygen that it can hold. During
the process of weathering fresh particles of rock are
continually being exposed to the atmosphere and
much of the ferrous oxide in the exposed parts
becomes oxidised into ferric oxide, the red oxide of
iron that is familiar to us as rust. It is the red ferric
oxide that gives the characteristic red or brown
colour to the weathered deposits.

The amount of oxygen that has been withdrawn
from the atmosphere by this process is very con-
siderable. The weathering process may be a slow
one, but it must be remembered that the total
thickness of the weathered deposits during geolo-
gical times amounts to at least three or four hundred
thousand feet. It has been estimated that the
amount of oxygen that has been abstracted in this
way from the Earth's atmosphere during geological
times is equal to about twice the quantity now
contained in the atmosphere. It is clear, therefore,
that some other process must be in operation, which
is continually replenishing the oxygen and making
good the loss. The vegetation over the surface of
the Earth provides the agency by means of which
this replenishment is brought about. The green
plant absorbs carbon dioxide from the air, as we
have already seen in Chapter II, and uses the
energy from sunlight to decompose it, the energy-

IOO LIFE ON OTHER WORLDS

transformer being the green colouring matter,
chlorophyll, contained in the plant cells. The
carbon is used to build up the complex organic
substances found in living plants, the oxygen being
returned to the atmosphere as a by-product.

We have, therefore, in continual operation, through
the agency of the vegetation over the Earth's sur-
face, the replenishment of the oxygen in the atmo-
sphere at the expense of the carbon dioxide. The
converse process is, however, also going on.
Through the decay of vegetable matter and other
organic materials, oxygen is absorbed and carbon
dioxide is liberated. This carbon dioxide is again
available for building up new plant cells. It may
seem that we are arguing in a circle and that with
two contrary processes in operation we have proved
nothing and have got no nearer to explaining why
the atmosphere now contains so much oxygen.
This would be so if there were an exact balance
between the two processes, but actually in geolo-
gical times they have not balanced. Whenever
organic matter is buried so that it cannot become
oxidised and decay there is a net gain of oxygen to
the atmosphere. Organic matter has in the past
been buried on a large scale and has provided the
coal measures and oil deposits of today. It seems
probable that the present abundant supply of
oxygen in the atmosphere has been provided at the
expense of the carbon dioxide that it formerly con-
tained, through the burial of the organic matter
that now provides us with coal and oil. It has been
estimated that if the coal, oil and other organic
deposits could be unburied and completely burned,

EVOLUTION OF THE ATMOSPHERE IOI

the whole of the oxygen in the atmosphere would
be used up.

The Earth's atmosphere has thus passed through
an evolution that is full of interest. The initial
atmosphere, rich in hydrogen and helium, was
largely lost whilst the Earth was still young and hot.
A new atmosphere was provided as the Earth
cooled. Still further cooling resulted in the forma-
tion of the oceans. Then, through the action of
vegetation, there has been the replacement of
carbon dioxide by oxygen; the change from an
atmosphere rich in carbon dioxide to an atmosphere
rich in oxygen is accounted for by the burial of
large quantities of vegetation, which have thereby
been preserved from decay.

This study of the evolution of the atmosphere of
the Earth will be helpful when we come to investi-
gate the atmospheres of the other planets. It sug-
gests that the values of the velocities of escape at the
present time will provide a guide to an estimate of
the maximum amount of atmosphere that a planet is
likely to have; we have obtained definite evidence
that the Earth must have lost much of its initial
atmosphere at a rapid rate whilst it was still hot;
the possibility of this initial rapid loss must be
borne in mind in the case of the other planets. We
have learnt also that an abundance of oxygen and
a scarcity of carbon dioxide is indicative of plentiful
vegetation, whereas, in the absence of vegetation,
we are likely to find an abundance of carbon
dioxide and a scarcity of oxygen.

CHAPTER V

WORLDS WITHOUT
ATMOSPHERES

WHEN the velocity of escape from a planet is suffi-
ciently small, the atmosphere will have been dissi-
pated at a rapid rate, so that we should not now
expect to find any traces of an atmosphere. These
conditions are likely to be found on the small bodies
of the solar system; for the velocity of escape is de-
termined by the ratio of the mass to the radius of the
planet, and though the mass is small for the planets
of small size and large for the planets of large size,
the mass is dependent upon the cube of the radius ;
it follows that, apart from differences in mean
density from one planet to another, the velocity of
escape is proportional to the radius of the planet.
It is, therefore, the small planets that will have
the low velocities of escape. The figures given on
p. 6 1 confirm this.

Of the bodies listed there, the Moon has the
lowest velocity of escape, viz. 1-5 miles a second.
The observed maximum temperature on the Moon
is I2OG.; at this temperature the average mole-
cular speeds are twenty per cent, greater than the
speeds at the temperature of o C., which are given
for a selection of gases on p. 56. The criterion for
the retention of an atmosphere tells us that at its
present temperature the Moon could retain carbon
dioxide and any heavier gases, but oxygen and all
lighter gases including nitrogen, water- vapour,

IO2

WORLDS WITHOUT ATMOSPHERES 103

helium and hydrogen would be lost. In the
earlier stages of the Moon's history, when it was
much hotter than it now is, the rate of escape of
atmosphere must have been very rapid. Even at
the present time the Moon would lose an atmo-
sphere of hydrogen almost instantly. At a tem-
perature of 1,000 C. it would lose an atmosphere
of carbon dioxide in a few years. We may expect,
therefore, to find that the Moon is now totally
devoid of any atmosphere.

The next lowest velocity of escape is that for
Mercury, 2-4 miles a second. The observed
average temperature of the sunlit side of Mercury is
about 400 C., and at this temperature the average
molecular velocities have 1-57 times their values at
o C. At such a temperature, Mercury could
retain atmospheres of carbon dioxide and oxygen
but not of lighter gases. But since the maximum
temperature on the sunlit side of the planet is appreci-
ably higher than 400 C., the chance of retaining
an atmosphere is less favourable than this suggests.
If Mercury had remained for long at much higher
temperatures before it cooled to its present state,
it must also, like the Moon, have lost its atmosphere
entirely. We can have little hope of finding any
evidence of an atmosphere on Mercury.

We come next to Mars, from which the velocity of
escape is 3-2 miles a second and whose temperature
is rather lower than that of the Earth. Mars could
not at the present time retain either hydrogen or
helium in its atmosphere, but it can retain water-
vapour and heavier gases. Admitting a rapid loss
of atmosphere in its early years, when its tempera-

IO4

LIFE ON OTHER WORLDS

ture was high, we must nevertheless conclude that
Mars is likely to have retained a certain amount of
atmosphere. The conditions are, however, some-
what critical, and Mars is barely able to retain an
atmosphere. It is a planet of particular interest
and will be considered in detail in a later chapter.

We have not hitherto made any mention of the
satellites of planets other than the Earth. Mars has
two satellites, Jupiter has eleven, Saturn has nine,
Uranus has four and Neptune has one. Are any of
these at all likely to possess an atmosphere ? They
differ considerably in size; some, including the two
satellites of Mars and the smallest satellites of
Jupiter's large family, are not more than a dozen or
so miles in diameter, whilst the largest slightly
exceed the planet Mercury in size. It is certain
that, with the possible exception of the few largest
satellites, it is quite impossible for them to have
retained any atmosphere at all.

Particulars of the largest satellites are as follows :

Planet.

Satellite.

Diameter
(Moon = i).

Mass
(Moon = i).

Velocity of ,
escape
miles/sec.

Jupiter

lo

I'D?

I-Og

i'5

Jupiter

Europa

O-gi

0-65

i'3

Jupiter

Ganymede

1-48

2-10

1-8

Jupiter

Gallisto

i'49

0-58

0-9

Saturn

Titan

I-2I

1-86

2'O

Neptune

(No name)

1-4?

?

?

The satellite of JSfeptune is too far away for its size
to be known with any accuracy, but from its bright-
ness it is estimated that it is not much inferior in

WORLDS WITHOUT ATMOSPHERES 105

size to Ganymede and Callisto, the largest of the
satellites of Jupiter. Nothing is yet known about
its mass.

The velocities of escape from these satellites,
given in the last column, may be compared with
the velocity of escape from the Moon, 1-5 miles a
second, and from Mercury, 2-4 miles a second. For
three of the satellites the velocities of escape are
equal to or are less than the velocity of escape from
the Moon ; for the other two, the velocities are inter-
mediate between the velocities of escape from the
Moon and Mercury. The temperatures of these
satellites are all very low, so that it would be pos-
sible for them now to retain atmospheres of the
heavier gases though gases lighter than water-
vapour could not be retained. But they will have
passed through a high- temperature stage, and if this
stage had lasted for some time they must have lost
their atmospheres entirely. If any of them has an
atmosphere at the present time it must be extremely
tenuous.

The conclusions that we have reached about the
atmospheres of the Moon and of Mercury have been
based entirely on theoretical considerations. It is
of interest to inquire whether these conclusions are
confirmed by observation. The Moon is our
nearest neighbour in space and can be studied in
greater detail than any other celestial body. Its
distance is only about a quarter of a million miles.
Such a distance may seem large when we compare
it with distances on the Earth; an aeroplane
travelling without any stop at a speed of two
hundred miles an hour would take seven weeks to

IO6 LIFE ON OTHER WORLDS

reach the Moon. But, in comparison with most of
the distances that are the concern of the astronomer,
the Moon can properly be described as a near
neighbour. It is so near that an object on the
Moon of the size of St. Paul's Cathedral could just
about be detected under favourable conditions.

In the telescope the Moon appears as a rugged
mountainous world. There are great mountain
ranges on the Moon, the finest being the Apennines,
shown in Plate 4, but the most prominent and
characteristic feature of its surface is the great
number of mountain rings, which cause large areas
of the surface to have a sort of pock-marked ap-
pearance. We are able to see only one half of the
surface of the Moon, because the Moon always
turns the same face towards the Earth, the other
face being permanently turned away from us. This
condition has been brought about through the
action of the gravitational attraction of the Earth.
When the Moon was young and its surface was still
plastic, the attraction of the Earth raised a great
tidal bulge on its surface; as the Moon rotated, this
tidal bulge was dragged across it so as always to
face towards the Earth. The dragging action
formed a brake on the rotation of the Moon and
gradually slowed it down until at length the Moon
turned always the same face towards the Earth.
An analogous effect is still slowing down the rotation
of the Earth. The tides in the oceans, produced by
the gravitational action of the Moon, act as a brake
upon the Earth, causing a very slow but progressive
lengthening in the day. The magnitude of the
effect is small, the length of the day increasing in

WORLDS WITHOUT ATMOSPHERES 107

the course of a century by only two-thousandths of
a second; but the effects are cumulative and will
continue until eventually the Earth will always
turn the same face to the Moon. When that hap-
pens the length of the day will have increased to
about forty-seven of our present days.

The ring mountain formations on the Moon are
commonly called craters, from their resemblance to
the volcanic craters on the surface of the Earth,
though they are on a much greater scale than the
largest terrestrial craters. The largest lunar craters
have a diameter of more than one hundred miles,
but there are many others that do not measure
more than a mile or two across. Small craters are
often found within larger ones and, in many cases,
the wall of one crater breaks through the wall of
another, suggesting that they were formed at
different times. The complexity of the topography
of the Moon, and the extreme ruggedness of its
surface, are well illustrated by the portion of the
surface shown in Plate 5. The designation of these
ring formations by the term crater is perhaps mislead-
ing, because it suggests a definite volcanic origin,
whereas it is by no means certain that they have
been formed by volcanic action. One theory of
their origin does suppose, however, that they are the
result of volcanic activity. Many of the craters
have a central peak and it is suggested that material
was ejected from a central volcano and gradually
built up the ring-shaped wall. The force of gravity
is much smaller on the Moon than on the Earth
and this would permit matter to be ejected without
difficulty to great distances. There is, however, no

IO8 LIFE ON OTHER WORLDS

essential difference in general structure between the
largest and the smallest craters, and it' is difficult to
understand how the largest craters, one hundred or
more miles in diameter, could have been produced
by volcanic action. Evidence of past volcanic
activity on the Moon seems to be provided, never-
theless, in another way, by the great plains or
" seas " as they are commonly called; it is
probable that these were once actual seas of
lava and irregular ripple-like markings on them
appear to indicate the limits of successive waves
of lava flow.

A different theory of the craters supposes that they
were formed as the result of the bombardment of
the Moon by gigantic meteors. There are a few
examples of meteor bombardment to be found on
the Earth's surface, the largest being the Great
Meteor Crater in Arizona, a hollow about 600 feet
deep and rather more than a mile in diameter,
with a raised rim. It is possible that in the early
stages of the Earth's history there was much more
material, which had not been aggregated into
planets, in the neighbourhood of the Sun than
there now is, and that both the Earth and the Moon
were subjected to intense meteoric bombardment.
The Moon still bears the marks of this bombard-
ment in its much-scarred surface; but the action of
denudation by water and erosion by wind, with the
formation of sedimentary deposits, and successive
upheavals to form mountain ranges have long ago
wiped away all the traces from the Earth's surface
of the bombardment that it had once undergone.
The few meteor craters that can be seen on the

WORLDS WITHOUT ATMOSPHERES

Earth at the present time are formations that are
geologically young.

Other formations of interest may be seen on the
surface of the Moon. There are many straight
clefts or crevasses, about half a mile or so in width,
which run in more or less straight lines for hundreds
of miles through valleys and across mountains.
They appear to be cracks in the surface, caused by
the contraction of the interior of the Moon. There
are also many deep narrow valleys and some straight
lines of cliff caused by faulting of the surface. The
most puzzling of all the features are the light-
coloured streaks or rays, which radiate radially
from some of the craters and pass across valleys
and mountains and over craters for distances of
hundreds of miles. They are a few miles in
breadth and do not appear to be elevated above
the surface. The true nature of these rays is not
known, through it has been suggested that the
appearance is caused by the staining of the surface
by vapours coming from narrow rifts. The rays
from the great crater Copernicus are shown in
Plate 6.

The appearance of the Moon in the telescope,
with its steep rugged mountains, jagged rocks and
fault scarps, suggests a world where there has been
no wearing away by erosion. It is quite certain
that the Moon is a waterless world. Oceans, lakes
and rivers would be clearly seen if they existed and
at times they would reflect the sunlight and appear
intensely bright. No clouds ever veil the Moon's
surface. This is merely what we should expect if,
as we have concluded, the Moon has no atmosphere.

IIO LIFE ON OTHER WORLDS

If there were any water on the Moon it would
rapidly evaporate during the heat of the long lunar
day and the water-vapour would be dissipated
away into space.

The absence of an atmosphere is indicated also
in other ways. The telescopic appearance of the
Moon itself proves that there cannot be more than
the merest trace of atmosphere, for the parts of the
Moon near the edge of the disk are as sharply
defined as the other parts of the surface; if there
were any atmosphere, the edge portions of the
Moon would be seen through a much greater depth
of atmosphere than the central portions and would
be partially or even completely obscured. Further
evidence is provided by observing the occultation
of a star as the Moon passes in front of it. The
Moon moves eastward across the sky relative to the
stars; between New Moon and Full Moon the
advancing edge is the dark one. When the dark
edge passes in front of a star, the star retains its
full brightness until it instantaneously disappears.
There is no gradual fading away as there would be
if the Moon had any atmosphere; at one moment
the star is seen to be shining with its usual bright-
ness; the next moment the star is not there. It is
as though it had suddenly been completely blotted
out of existence. And however often the observa-
tion is made, the suddenness of the disappearance
never ceases to cause surprise. Between Full Moon
and New Moon the advancing edge is the bright
one; a star that has been occulted then reappears
with equally startling suddenness as the dark edge
passes away from it.

WORLDS WITHOUT ATMOSPHERES III

We see the Moon by means of sunlight falling
upon it which is reflected back by its surface. At
any time one half of the surface is in sunlight and
the other half in darkness. Near New Moon the
sunlight is mostly falling on the hemisphere of the
Moon that is turned away from us; near Full Moon
it is mostly falling on the hemisphere that is turned
towards us. This is the cause of the varying phases
of the Moon. Though the Moon appears very
bright, its surface is actually a poor reflector; less
than ten per cent, of the sunlight that falls on it
is reflected back, the remainder being absorbed
and going to heat the surface. The reflecting
power of the surface of the Moon corresponds to
that of greyish-brown rocks. It may be remarked
that the greyish colour of the Moon is suggestive
of rocks that are incompletely oxidised and indicate
the absence of oxygen; if oxygen were present on
the Moon to oxidise the rocks completely, the
colour of the Moon would be reddish, somewhat
resembling the colour of Mars.

The temperature of the Moon can be measured
with the aid of a bolometer or thermocouple, as
previously described. As we have just mentioned,
most of the radiation that we receive from the Moon
is merely reflected sunlight. The remainder is heat
radiation radiation that has been absorbed by the
surface rocks, which become heated and radiate like
a brick wall that has been heated in the Sun. This
heat radiation, characterised by its long wave-
length, can be separated from the reflected sunlight,
which is mainly of much shorter wave-length, and
used to determine the surface temperature. During

112 LIFE ON OTHER WORLDS

the long lunar day, equal to fourteen of our days,
the surface rocks become very hot and the tem-
perature near the equator of the Moon at noon is
about I20C., considerably higher than the tem-
perature of boiling water at the surface of the
Earth (100 C.). The temperature falls rapidly as
the altitude of the Sun decreases and by sunset it
is already freezing hard, the temperature being
then 10 G. (or 18 of frost on the Fahrenheit
scale). The most rapid variations of temperature
occur at the time of an eclipse of the Moon. As
the Earth advances, throwing its shadow on the
Moon and cutting off the sunlight from its surface,
the temperature immediately begins to drop with
startling rapidity. At one eclipse, for instance, it
was found that the temperature of the surface
dropped from +70 C. to 80 G. in a little more
than an hour. During totality, which lasted 2^
hours, the temperature dropped a further 40 C.
But after totality had ended, the temperature rose
in an hour to almost its initial value. These
extreme and rapid variations are what we should
expect on a world that is entirely devoid of at-
mosphere. We may compare these figures with
the fall of temperature experienced on the Earth
at points within the belt of totality on the occasion
of a total eclipse of the Sun; the fall of temperature
does not usually exceed two or three degrees.

From what we have learnt of the Moon, and in
particular from its lack of oxygen and water and
from its extreme variations of temperature, we
should naturally conclude that it is a world where
life of any sort is entirely out of question. The

WORLDS WITHOUT ATMOSPHERES 113

Moon is the only world where we should expect
actually to see clear evidence of life, if any existed.
There is no doubt that if there were a lunar inhabi-
tant equipped with a powerful telescope, he would
be able to see many signs of human activity on the
Earth. He would be able to watch the growth of
greater London; he would see cities like New York,
Sydney, Johannesburg and Ottawa springing up.
He would be able to watch the formation of new
lakes by the impounding of water by dams. He
would see land-reclamation works in progress and
the draining of the water from tracts such as the
Zuyder Zee. The seasonal growth of vegetation
and the melting of the snow over vast tracts of land
with the advance of summer would be clearly
visible to him. In the course of a few years he would
undoubtedly obtain clear evidence not only of
plant life on the Earth but also of human activity.
So, in a similar way, if the Moon were inhabited
by intelligent beings we should expect to find plenty
of evidence of their existence. We can find none.
The Moon shows no signs of change. There are
not even seasonal changes of colouration, such as
might be attributed to the growth of vegetation.
Some astronomers have claimed, indeed, to have
seen slight changes in certain regions, changes
mainly of tint such as might be produced by the
growth of lichens on rocks. But such changes have
not been confirmed and are generally discredited.
It seems that the observers have been misled by
changes in the appearance of the surface detail
with changing altitude of the Sun. No! it is not
possible to admit that there is life of any sort on the
8

114 LIFE ON OTHER WORLDS

Moon. It is a world that is completely and utterly
dead, a sterile mountainous waste on which during
the heat of the day the sun blazes down with
relentless fury, but where during the long night
the cold is so intense that it far surpasses anything
ever experienced on the Earth.

These hard facts are conveniently ignored by
those who believe that it would be possible to shoot
a rocket, containing human beings, to the Moon
from which the human explorers could land and
explore some portion of the Moon's surface. The
explorers would need to be encased in airtight suits
and provided with oxygen apparatus to enable them
to breathe. Even supposing that they could protect
themselves against the great heat by day and the
extreme cold at night, a worse fate might be in store
for them unless their suits were completely bullet
proof. For they would be in danger of being shot
by a shooting star. The average shooting star or
meteor, which gives so strongly the impression of a
star falling from the sky, is a small fragment of
matter, usually smaller than a pea and often no
larger than a grain of sand. Space is not empty but
contains great numbers of such fragments. The
Earth, in its motion round the Sun, meets many of
these fragments, which enter the atmosphere at a
speed many times greater than that of a rifle bullet.
The meteor, rushing through the air, becomes
intensely heated by friction and is usually com-
pletely vaporised before it has penetrated within a
distance of twenty miles from the surface of the
Earth. Many millions of these fragments enter our
atmosphere in the course of a day, but the atmo-

WORLDS WITHOUT ATMOSPHERES 115

sphere protects us from them. On the Moon,
however, they fall to the surface and so great is their
number that the lunar explorers would run a con-
siderable risk of being hit.

The difficulties that would have to be encountered
by anyone who attempted to explore the Moon
assuming that it was possible to get there would be
incomparably greater than those that have to be
faced in the endeavour to reach the summit of
Mount Everest. In two respects only would the
lunar explorer have the advantage. In the first
place movement would be less fatiguing because
as the weight of the Moon is only about one-eightieth
of that of the Earth the gravitational pull of fhe
Moon is not very great. If the Moon had an
atmosphere like that of the Earth, a golfer on the
Moon would find that he could drive his ball for a
mile without much difficulty and a moderate bats-
man would hit sixes with the greatest of ease and
perform feats that even Bradman might envy.
The second advantage the lunar explorer would
have over the climbers on Mount Everest would be
the absence of strong winds to contend against.
The Moon having no atmosphere, there can be no
wind ; nor, of course, can there be any noise, for
sound is carried by the air. The Moon is a world
that is completely still and where silence, " a
silence where no sound may be," prevails.

We turn next to the planet Mercury, the nearest
of the planets to the Sun. The year on Mercury,
the time that is taken for Mercury to traverse its
orbit around the Sun, is 88 of our days. The orbit
of Mercury is much the most elliptical of any of the

Il6 LIFE ON OTHER WORLDS

planets/ so that the distance of Mercury from the
Sun varies rather widely in the course of its passage
around the Sun. It ranges from about 28| million
miles, when at its nearest, to about 43^ million miles
when at its greatest distance. Mercury is the fastest
moving of all the planets, its speed ranging from
thirty-six miles a second, when it is at its nearest to
the Sun, to twenty-four miles a second, when
farthest away. It is, therefore, not inappropriately
named after the winged-footed messenger of the
ancient gods.

Mercury turns always the same face towards the
Sun, just as the Moon turns always the same face
towards the Earth. In the course of its journey
round the Sun, it therefore makes exactly one rota-
tion about its axis. In other words, the length of
the day (defined as the time taken by the planet to
make one rotation about its axis) is equal to the
length of the year. But the other use commonly
made of the word day, as contrasted with night,
ceases to have any meaning on Mercury. For a
portion of the planet has perpetual day and
another portion has perpetual night. If Mercury
were always at the same distance from the Sun,
exactly half of the planet would have perpetual day-
time and the other half would have perpetual night-
time. This condition is modified somewhat by the
variation in the distance of Mercury from the Sun.
About three-eighths of the surface has perpetual
day and an equal area has perpetual night. In the

1 We leave out of consideration the thousands of minor planets or
asteroids which circulate round the Sun between the orbits of Mars
and Jupiter.

WORLDS WITHOUT ATMOSPHERES 117

intermediate zone, embracing about one-quarter of
the surface, the Sun alternately rises above the
horizon and falls below it.

The temperature of the portion of Mercury that is
perpetually bathed in sunlight is very high, about
400 C., a temperature considerably higher than
that at which lead melts and close to the tempera-
ture required for zinc to melt. The opposite side,
which never receives any sunlight at all, must be
intensely cold. In the zone between, which has
alternately sunlight and darkness, the variations of
temperature must be extremely great.

We have seen that there is little expectation of any
atmosphere on Mercury. Though, at the present
time, Mercury could hold an atmosphere com-
posed of the heavier gases, the atmosphere must
have escaped entirely if Mercury had remained very
hot for any length of time after its formation. We
have already concluded that the Earth, where
conditions are much less favourable for the escape
of an atmosphere, must have lost most of its original
atmosphere whilst it was still hot. It is just possible
that though Mercury lost all its original atmosphere,
gases were given off by the crust as it solidified and,
if so, Mercury may possibly now have an extremely
tenuous atmosphere of carbon dioxide.

It is not easy to find out anything very definite
about conditions on Mercury by direct observation.
Being near the Sun, Mercury is never visible for
very long after sunset or before sunrise and it is
comparatively seldom seen with the naked eye.
The great astronomer, Copernicus, died without
ever having seen it; he did not have the advan-

Il8 LIFE ON OTHER WORLDS

tage of the optical aid provided by telescopes,
which makes it possible for Mercury to be seen in
broad daylight. But broad daylight does not
usually provide favourable conditions for attempting
to see any markings on the surface of Mercury.

Exceptionally steady atmospheric conditions are
required for the study of the surface of Mercury.
In 1882 an Italian astronomer, Schiaparelli, began
to make a systematic study of this planet, and in
1889 he announced that there were markings on the
surface, which were of a permanent nature and
which indicated that Mercury always turned the
same face to the Sun. The markings were ill-
defined and gave the surface a spotty appearance.
Subsequent observations by other observers have
confirmed Schiaparelli's observations in their broad
essentials, though there are considerable differences
between the markings recorded by different ob-
servers. This is not surprising, for the observations
are of extreme difficulty. There is, nevertheless,
general agreement about the main features, and it
seems reasonably certain that the markings are due
to irregularities of the surface as in the case of the
Moon. It is possible that the surface of Mercury
bears a general resemblance to the mountainous
surface of the Moon.

Difficult as it is to observe the surface markings of
Mercury, it is far more difficult to obtain any certain
evidence of an atmosphere. The reflecting power
of Mercury, is about equal to that of the Moon. If
there was much atmosphere, Mercury would un-
doubtedly reflect a good deal more of the sunlight
that falls upon it. Observations of the changes of

WORLDS WITHOUT ATMOSPHERES Iig

brightness of the planet with changes of phase
suggest that there can be little or no atmosphere,
and that the light is reflected from a rough surface,
like that of the Moon.

It sometimes happens that Mercury passes
directly between the Earth and the Sun; it may
then be seen projected on the Sun's disk as a small
round black spot. The moments when it is entering
on to, or leaving, the disk of the Sun provide
favourable opportunities for detecting any atmo-
sphere. At such times, if there were an appreciable
atmosphere on Mercury, the portion of its disk
outside the Sun would be surrounded by a bright
line or halo, produced by the refraction of sunlight
through the planet's atmosphere. Such a bright
halo is seen when Venus is entering upon the Sun's
disk, but it is not noticed in the case of Mercury.
This observation provides definite evidence that,
if there is any atmosphere on Mercury, it must be
of extreme rarity.

Some observers have felt confident, neverthe-
less, that Mercury must have a thin atmosphere
because, they assert, certain of the surface markings
are obscured from time to time. The temperature
is too high for any sort of condensation cloud to form
above the surface and it has accordingly been
suggested that the obscuration is caused by dust.
It is possible that there may be active volcanoes on
Mercury which eject clouds of dust to a great
height at times of violent eruptions, and that there is
a very thin atmosphere, sufficient to prevent the
fine dust settling too rapidly. More will be
learned in the future, we may hope, about this.

I2O LIFE ON OTHER WORLDS

The observations are extremely difficult and are not
free from doubt. They need further confirmation.
At present all that we can positively assert is that
Mercury can have at the most an atmosphere of
extreme rarity, and that the existence of such an
atmosphere is not impossible on theoretical grounds.

Whatever the final conclusion about the atmo-
sphere of Mercury may be, there seems to be no
possibility that life can exist on the planet. The
high temperature over much of its surface, the very
low temperature over other parts and the extreme
variations of temperature over the intermediate
zone, together with the absence of water-vapour and
oxygen which must have been lost whilst Mercury
was much hotter than it is at present combine to
make conditions under which no form of life could
survive.

The two largest satellites of Jupiter, Ganymede
and Callisto, are larger than the planet Mercury;
the other two major satellites of Jupiter, lo and
Europa, the largest of the satellites of Saturn, Titan,
and the satellite of Neptune though smaller than
Mercury are larger than the Moon. The velocities
of escape from these bodies do not differ greatly
from the velocity of escape from the Moon, and at
their present temperatures they could just about
retain water-vapour and heavier gases and vapours.
But it is not possible to suppose that they have been
very cold throughout their history and it is probable
that their atmospheres were entirely or almost
entirely lost whilst they were young and hot.

Surface markings can be seen on some of these
bodies and they all appear to turn the same face

WORLDS WITHOUT ATMOSPHERES 121

continually to the parent planet. They all have
rather low mean densities; lo and Europa have
mean densities nearly three times that of water;
these two satellites are probably masses of rock like
the moon. The mean density of Gannymede is
slightly more than twice that of water, whilst that
of Gallisto is only six-tenths that of water. These
two bodies may consist largely of rock with a thick
covering of ice or solid carbon dioxide. The very
low mean density of Callisto may be spurious, as the
mass of this satellite is difficult to determine and is
somewhat uncertain; if real, it is difficult to explain
unless we may suppose that its rocks are mainly in
the form of pumice.

The average temperatures of the satellites will
not differ much from those of the parent planets
because a planet and its satellites are at approxi-
mately the same distance from the Sun. Reference
to the table of temperatures on p. 76 shows that
the large planets are extremely cold. These low
temperatures would render the development of any
form of life impossible. We may conclude that it is
unlikely that any of the satellites in the solar system
have an atmosphere; but in those cases where there
may remain some doubt about the correctness of
this assertion, the temperatures are certainly too
low for the existence of life. Hence we can leave
all the satellites out of consideration in our search
for conditions in the solar system where life might
conceivably exist.

CHAPTER VI

THE GIANT PLANETS

THE four major planets, Jupiter, Saturn, Uranus
and Neptune, are much larger and much more
massive than the Earth. Their radii and masses,
relative to the corresponding quantities for the
Earth, are given in the table on p. 61. The same
table gives the velocities of escape from these bodies.
These velocities are all high and are, in fact, so
much greater than the average molecular velocity
of hydrogen that it is impossible for hydrogen, and
therefore also for any of the heavier gases, to have
escaped from their atmospheres, even if we suppose
that they were initially much hotter than they now
are. The giant planets must consequently have
retained their initial atmospheres in their entirety.
We may accordingly expect to find that they all
possess dense and extensive atmospheres. For this
reason they have many similarities, and it is
convenient to consider them together.

The nearest and the largest of these four planets
is Jupiter, which is, therefore, the most favourably
placed for detailed observation and study with the
telescope. A small telescope is sufficient to show
Jupiter as a fine-looking object. The apparent angular
diameter of Jupiter varies from thirty- two seconds
of arc, when it is at its greatest distance from the
Earth (600 million miles), to fifty- two seconds when
it is at its nearest (367 million miles). Even when it
is at its greatest distance, a low magnification of

122

THE GIANT PLANETS 123

sixty will make it appear in the telescope as large
as the Moon appears to the naked eye. We sup-
pose, then, that we are looking at Jupiter in a
moderate-sized telescope. We see a bright disk,
crossed by a number of dark markings arranged in
more or less parallel belts. The disk is not circular
in outline, but is distinctly flattened, the shortest
axis being at right angles to the parallel belts. This
flattening suggests a body that is in rapid rotation.
If the flattening is produced by rotation, the shortest
axis will be the axis of rotation; the belts are there-
fore parallel to the equator of Jupiter. When we
look attentively at the dark markings, we notice
that they are not of a uniform structure and of
sharply defined outline but that they show great
complexity of structure and variety of detail.
Fixing attention on any convenient well-defined
object on the surface of the planet, we notice that it
appears to move gradually across the surface. ^ This
apparent motion is produced by the rotation of
Jupiter on its axis. If we watched for a sufficiently
long time, we should see the object disappear at one
limb of Jupiter and after several hours reappear at
the opposite limb; in rather less than ten hours
from the commencement of the watch we should
find that the object had returned to the same posi-
tion on the disk that it occupied when the observa-
tions commenced. We thus confirm the rapid
rotation of Jupiter, which the flattened shape of the
planet had caused us to suspect, and we find that
the time of rotation is rather less than ten hours.
Jupiter has, in fact, the shortest period of rotation
of any of the planets.

124 LIFE ON OTHER WORLDS

If our observations are made in the twilight,
before the sky has become dark, we will notice that
the centre of the disk is much brighter than the
edge. This effect is much less noticeable at night,
when the sky is dark, because the contrast in
brightness between the limb and the dark sky then
makes the edge portions of the planet appear sub-
jectively relatively brighter than they really are.
Precise measures of the brightness with a photo-
meter show that the brightness begins to fall off
rapidly near the edge of the disk and that at the
edge the brightness is only about one-eighth that at
the centre. This suggests that Jupiter has an atmo-
sphere and that the falling off in brightness towards
the edge is produced by the absorption of light in
this atmosphere. In the case of the Sun, which is
a gaseous body, there is a similar decrease in
brightness from the centre to the edge of the disk,
whereas the Moon, which is a solid body without
an atmosphere, does not show any difference in
brightness between centre and edge.

If we were to embark on regular systematic
observations of Jupiter we should soon find that
the phenomena shown by the surface markings are
of considerable complexity. It was noticed as long
ago as the seventeenth century that the rotation
period was not the same for all portions of the
planet, the equatorial regions rotating faster than
the polar regions. In other words, Jupiter is not
spinning round on its axis like a solid body; this
provides further evidence that the visible layers are
not solid but that they must be gaseous or, possibly,
liquid. A similar lack of uniformity in its rotation

THE GIANT PLANETS 125

is shown by the Sun, whose equatorial regions rotate
considerably more rapidly than the polar regions.
The period of rotation of the Sun at the equator is
about 24! days, whilst at the poles it is about 34.
days. But whereas the period of rotation of the
Sun increases progressively from equator to poles,
there is no corresponding regularity in the case of
Jupiter. There are a number of different zones,
each of which is pretty sharply defined and each of
which has its own rate of rotation, with abrupt dis-
continuities from one zone to the adjacent zones.

The pioneer work in the systematic detailed study
of these movements was made by a British amateur
astronomer, Mr. Stanley Williams, who died in
1938. He established the existence of a number of
definite currents in the surface material; these cur-
rents determine the rotation periods of the spots
or markings that appear within them. Williams
showed further that the currents were of a perman-
ent nature, although from time to time there were
temporary variations in the rates of their movement.
The results obtained by Williams gave a great
impetus to the study of the surface markings of
Jupiter and the planet has been kept under system-
atic observation by the devoted band of amateur
observers that forms the Jupiter Section of the
British Astronomical Association, pre-eminent
amongst whom is the Rev. T. E. R. Phillips, who
directed the work for more than thirty years. Two
drawings of Jupiter by Mr. Phillips are reproduced
in Plate 7, along with two photographs taken with
the 36-inch refracting telescope of the Lick Observa-
tory.

126 LIFE ON OTHER WORLDS

These observations have confirmed the perman-
ence of the eleven currents found by Williams.
The most important of these is the great equatorial
current, which covers a zone from 10,000 to 15,000
miles wide and has a period of rotation of rather
more than 9 hours 50 minutes. The periods of
rotation of the other currents lie between 9 hours
55 minutes and 9 hours 56 minutes, but bear no
relation to the latitude. The distribution of the
currents is different in the two hemispheres. The
differences in period of rotation of the several cur-
rents may seem to be not very large and they are,
of course, very much smaller than the differences in
the rates of rotation of the various portions of the
Sun. But the differences are perfectly distinct and
definite, so that well-marked features in adjacent
currents may drift past one another, as the result of
the different rates of movement of these currents, at
a relative speed of as much as two hundred miles
an hour.

Though the different currents are permanent,
their positions as well as their rates of rotation may
vary from year to year. These variations have been
well established by the observations of Williams,
Phillips and others and are not to be attributed to
errors of observation. The movement of the cur-
rents and of the spots or other markings associated
with them is usually almost entirely in a direction
parallel to the equator, though from time to time
spots have been observed whose movement is of a
circulatory nature.

Most of the markings to be seen on Jupiter are
short lived. Some are merely transient, others may

THE GIANT PLANETS 127

lafct for a few weeks or, occasionally, for a few months.
Their shapes show continual change. There can
be little doubt that they are atmospheric pheno-
mena of some sort. Their appearance suggests
that they may be clouds, formed of droplets of
condensed vapours, floating in the atmosphere of
Jupiter.

There are a few markings, however, that are of a
much more permanent nature. The most remark-
able of these is the great Red Spot. This marking
was seen in 1878, and has been visible ever since,
though its appearance changes so much that at
some times it is very conspicuous and of a striking
brick-red colour, whilst at other times it loses
its colour and appears to fade away until it can
scarcely be seen. It is normally oval in shape,
extending some 30,000 miles in length and with a
width of about 7,000 miles, the long axis being
parallel to the equator; from time to time the shape
changes and it becomes rounder. From a study of
earlier drawings and observations of Jupiter it has
been concluded that it was observed as far back as
1831 and that it may well be identical with a mark-
ing observed by Hooke in 1664. The appearance
in 1878 came after a period of relative faintness. It
was at one time thought that the Red Spot was a
portion of the surface of Jupiter seen through a
break in the clouds or that it was in some way
attached or related to the solid surface; it has been
suggested, for instance, that it might be a cloud or
pall of smoke and dust hanging over an active
volcano on Jupiter. But it has been found that
the period of its rotation is subject to large and

128 LIFE ON OTHER WORLDS

irregular variations, so that it is impossible that
it can be related to the solid surface of the
planet.

Another remarkable marking is known as the
South Tropical Disturbance. It is a dark region,
some 45,000 miles in length, lying in the belt just
south of the Red Spot, which was first seen in 1901.
Its rotation is somewhat more rapid than that of
the Red Spot, so that it overtakes the Spot at inter-
vals which were at first about two years in length
but which have gradually become longer, streaming
past it at a relative rate of several miles an hour.
When the disturbance is overtaking the Spot, its
motion becomes accelerated and, in passing the
Spot, it tends to drag the Spot along with it for
several thousands of miles, the Spot then drifting
back to its previous position.

The belts on Jupiter are rich in colour, the colours
being sometimes very vivid. Though red, brown
and orange colours predominate, olive green and
bluish patches may also be seen. The colouration
of the belts is variable but the two hemispheres of
the planet usually vary in an opposite manner.
When the belts in one hemisphere show a maximum
of redness, those in the other hemisphere are
colourless or slightly bluish and conversely, whilst
at intermediate times the predominant effect in
both hemispheres is a moderate redness.

It was thought until within fairly recent years
that Jupiter was almost red-hot and that the bright
colours seen in the belts were due to glowing
vapours rising into the atmosphere from the almost
molten mass. It was known, however, that Jupiter

THE GIANT PLANETS

was not sufficiently hot to be perceptibly self-
luminous ; for from time to time one or other of the
satellites passes between the planet and the Sun
and, when this happens, the shadow of the planet
is cast upon the surface; these shadows appear quite
black, which they would not do if there was any
appreciable glow from the surface. Observations
within the last twenty-five years have proved that the
belief that Jupiter is hot was ill-founded. Direct
measurements have shown that most of the radiation
that reaches us from Jupiter is merely reflected
sunlight; the small portion that is true planetary
radiation indicates that the temperature of the
surface is very low, about 140 C. This tempera-
ture is in close agreement with the temperature
that would be expected for a planet at the distance
of Jupiter from the Sun, if the surface were warmed
only by the radiation from the Sun. It can be
inferred that there is a close balance between the
radiation that Jupiter receives from the Sun and
the radiation that it re-emits to space, leaving little
or no residuum to be accounted for by heat coming
from the interior of the planet.

The appearance of Saturn in the telescope,
leaving the rings out of consideration, is generally
similar to that of Jupiter. The surface is marked
by bright and dark belts parallel to the equator.
But whereas the belts of Jupiter are very irregular
in outline, show a great deal of detail in the form
of bright and dark spots, and undergo rapid
changes, the belts of Saturn are regular in outline,
are ill-defined and show little detail and have few
spots or other irregularities. Information about

130 LIFE ON OTHER WORLDS

the rotation period of Saturn and about the variation
of the rotation with latitude is therefore not readily
obtainable. From time to time, however, white
spots appear, which may persist for some time. In
1794 Sir William Herschel observed such a spot
and was able to determine its period of rotation,
which he found to be ten hours sixteen, minutes;
the rotation of Saturn is therefore slightly slower
than that of Jupiter. Other spots have been ob-
served from time to time, at rather infrequent
intervals. The most conspicuous spot ever observed
on Saturn appeared in 1933 and was first seen by
Mr. Will Hay in London. This spot, like the spot
observed by Herschel in 1 794, was near the equator
and the rotation period of the two spots were in
close agreement. As with Jupiter, the higher the
latitude the slower is the rotation, though the
change with latitude is much greater for Saturn than
for Jupiter. A spot discovered by Barnard in 1903
in latitude 36 north gave a period of ten hours
thirty-eight minutes.

As in the case of Jupiter, the edges of the disk of
Saturn are much less bright than the centre,
suggesting the presence of a considerable atmo-
sphere. The disk is also strikingly flattened, the
axis of rotation being the short axis. The flattening
is more marked than for Jupiter; Saturn is, in fact,
much the most flattened of all the planets. This is
another indication of an extensive atmosphere.
Saturn shows colours which are not so strongly
marked as those of Jupiter. The equatorial zone
is usually of a bright yellow colour and there is a
darkish cap, of a greenish hue, at the pole. The

THE GIANT PLANETS 131

measured temperature of Saturn is very low,
155 C., some 15 C. lower than that of Jupiter,
as would be expected from its greater distance from
the Sun.

It is the rings of Saturn that make it such a unique
and striking object in the telescope. In the dis-
cussion of conditions on Saturn, in relation to the
possibility of the existence of life on the planet,
we are not really concerned with the rings, which
are not solid and could not be the home of life.
It was shown in 1857 ^Y Clerk Maxwell that the
rings could not continue to exist unless they were
composed of a multitude of small particles : if the
rings were solid, liquid or gaseous they would be
unstable and would break up. The rings may
therefore be considered as consisting of a great
number of tiny moons, circulating around Saturn.
The rule of the road that governs such motion
under the controlling force of gravitation is that
the outer lanes of traffic move more slowly than
the inner lanes. That this is actually the case was
proved by observation in 1895 by Keeler. The
rings naturally do not show any marks that can be
identified and used to serve as a measure of the
rate of rotation; but the frequency or wave-length
of the light received from a moving source depends
upon its velocity and Keeler used this fact to
investigate the rotation of the rings. There is little
doubt that the fragments of which the ring system
is composed are the remnants of a former satellite
of Saturn, which approached too close to the planet
and paid the penalty, being disrupted by the
gravitational pull of Saturn.

132 LIFE ON OTHER WORLDS

The remaining two major planets, Uranus and
Neptune, are at such great distances from the Sun
that we cannot learn much about them by direct
telescopic observation. Uranus, at its mean dis-
tance, has an angular diameter of nearly four
seconds of arc, which is equal to the angular
diameter of a halfpenny placed at a little less than
a mile away. The angular diameter of Neptune is
about half that of Uranus. Uranus can be seen
with the naked eye on a clear dark night, appearing
as a star of the sixth magnitude, but Neptune is
too faint to be seen without telescopic aid. In the
telescope both planets show small disks, which are
of a sea-green colour. The disk of Uranus shows
a decided oblateness; that of Neptune appears
sensibly round. Extremely faint belts, parallel to
the equator, have been seen on Uranus by several
observers under particularly favourable conditions;
they are suggestive of the belts of Saturn seen from
a very great distance. There are no markings
distinct enough to give a measure of the rotation
period, but spectroscopic observations have given
a value of above lof hours. This is probably a
pretty correct value because it has been found that
the brightness of Uranus is slightly variable in a
period of ten hours forty minutes. The variations
in brightness therefore synchronise with the rotation
of the planet and suggest that if we could observe
Uranus from a closer distance we should see marked
differences in the belt structure in different longi-
tudes. Neptune has a slower rotation period of
fifteen hours forty minutes; it has the slowest
rotation of any of the major planets. This rotation

THE GIANT PLANETS 133

period was measured by the spectroscopic method.
Observations of the brightness of Neptune had
shown that there was a small regular variation in
about half this time seven hours fifty minutes. So
short a period of rotation was difficult to fit in with
some theoretical considerations of the orbit of the
satellite of Neptune. It is now clear that there
must be two portions of the surface of the planet,
situated more or less on diametrically opposite sides
of the planet, that are brighter than the remainder.
From the same theoretical considerations the
flattening of the planet can be inferred. It is found
to be appreciable, though less than for the other
major planets. The little that we can learn by
direct observation about Uranus and Neptune
suggests, as we should have anticipated, that there
is a general similarity between these two distant
planets and the two nearer major planets, Jupiter
and Saturn. Measurement of the temperature of
Uranus indicates that it is below 1 80 G.;
Neptune must be still colder, but its temperature
has not been determined by direct observation.

The major planets can be weighed with consider-
able accuracy because they all have one or more
satellites. All that is required for this purpose is to
know the period of revolution of a satellite about
its parent planet and its distance from the planet;
the universal law of gravitation enables us to infer
the weight of the planet. Knowing also the size
of the planet we can infer its mean density. In all
the four major planets the mean density comes out
surprisingly low. The mean density of the Earth
is 5^ times that of water; the mean density of the

134 LIFE ON OTHER WORLDS

Moon is less, being 3^ times that of water and
suggesting that the Moon may have been formed
from the less dense outer portions of the Earth.
Some people believe that the Pacific Ocean, which
is by far the most extensive of the oceans, marks the
region where the Moon separated from the Earth.
This theory must be looked upon with considerable
suspicion, for there are very strong theoretical argu-
ments which suggest that the Earth and the Moon
could never have formed one body. The mean
densities of Mercury, Venus and Mars all lie between
the values for the Earth and the Moon. But when
we come to the major planets we find appreciably
lower values : the mean densities, in terms of that
of water, for Jupiter, Saturn, Uranus and Neptune
are respectively 1*34, 0-71, 1-27, 1*58. The low
density of Saturn, less than three-quarters the
density of water, is particularly surprising; it is only
half the mean density of the Sun, which is a gaseous
body. The mean densities being so much lower
than the densities of any rocks and the planets being
known to possess extensive atmospheres, the obvious
explanation is that these atmospheres must have
very great depth. An appreciable portion of the
apparent diameter of these planets is consequently
contributed by their deep atmospheres. Our ex-
pectation that these massive planets must have
extensive atmospheres is thus confirmed.

The temperatures of the giant planets are
extremely low. The moisture that must have been
present in their atmospheres, when their tempera-
tures were much higher than they are now, must
have condensed to form oceans as the planets cooled.

THE GIANT PLANETS 135

At a later stage, with still further cooling, these
oceans froze to form a coating of ice over the surface.
It was suggested by Dr. Jeffreys that these planets
may be regarded as consisting essentially of a core
of rock, generally similar to the Earth in its con-
stitution and of about the same mean density,
surrounded by ice-coatings of great depth, above
which are very extensive atmospheres. We are
unable to penetrate the atmospheres nearly far
enough to see these ice-coatings but we can be pretty
certain that they exist.

If, then, we picture these planets to be composed
of three regions, each sharply differentiated in
density the central rock core, the ice-coating of
lower density and the atmosphere of still lower
density it is possible to make approximate estimates
of the extent of each region and of the mean
densities of the atmospheres. These estimates ha.ve
to be determined to give the correct mean density
and the observed degree of flattening. According
to the calculations made by Dr. Rupert Wildt, the
rocky core of Jupiter has a radius of about 22,000
miles, so that it occupies only about one-eighth of
the whole volume corresponding to the visible disk.
The ice-coating is about 16,000 miles in thickness
and the depth of the atmosphere is about 6,000
miles. The figures for Saturn are even more
astonishing. The rocky core of Saturn is about
14,000 miles in radius; it is covered with a layer of
ice some 6,000 miles thick, over which is an atmo-
sphere extending to a height of 16,000 miles.
Saturn has the most extensive atmosphere of any of
the planets, both absolutely and relative to the size

136 LIFE ON OTHER WORLDS

of the planet; this explains why it has the lowest
mean density and the most flattened disk of any
planet. It is of interest to note that the total weight
of the atmosphere of Saturn is about equal to that
of its rocky core. The data for the more distant
planets, Uranus and Neptune, are a little more
uncertain; the glacier coating on both of these
planets is some 6,000 miles in thickness, whilst the
depths of the atmospheres are approximately 3,000
miles for Uranus and 2,000 miles for Neptune.
These figures must be regarded as liable to some
uncertainty, but there seems to be little doubt that
they provide a substantially correct picture of the
constitution of the giant planets.

Some interesting consequences follow from the
great extent of these atmospheres. The pressures at
the bottom of them are very great. At the bottom of
the atmosphere of Jupiter, for instance, the pressure
is fully a million times the pressure at the bottom of
the Earth's atmosphere or, in other words, at the
surface of the Earth. At a relatively small depth in
the atmospheres, the pressure is great enough to
compress each gaseous constituent of the atmosphere
to a density nearly equal to that of the corresponding
liquid or solid. It is stated by Wildt that at the
bottom of the atmospheres the pressure is great
enough to solidify even the permanent gases, includ-
ing hydrogen and helium. Little or nothing is
known about the properties of substances at these
enormous pressures; if we knew more we should
probably be in a better position to understand some
of the many puzzling phenomena that are presented
to us by the atmosphere of Jupiter, phenomena

THE GIANT PLANETS 137

that have been studied in some detail, as briefly
described previously.

We have spoken about extensive atmospheres to
the giant planets but we have now reached the sur-
prising conclusion that at a relatively small depth in
the outer layer of tow density the pressure becomes
sufficiently great to compress the constituents into
the solid or liquid state. The use of the term
atmosphere in connection with these planets is,
therefore^ somewhat misleading, and it would be
more correct to speak of the outer layer of low
density; this outer layer probably ceases to be
gaseous at a depth of not more than a few hundred
miles.

The mean densities of the outer layers are low;
according to Wildt's calculations they are 0-78 for
Jupiter and 0-41 for Saturn. This enables most of
the possible constituents to be excluded, for all
known gases in the solid or liquid state have densi-
ties exceeding 0-3, with the exception of hydrogen
and helium. Frozen oxygen, for instance, has a
density of 1^45; nitrogen, 1-02; ammonia, 0-82.
In addition to helium and hydrogen, the only gases
whose densities in the liquid or solid state are lower
than the density of the greater portion of the outer
layer of Jupiter are the two hydrocarbons, methane
or marsh-gas and ethane. There seems to be no
escape from the conclusion that the outer layer of
the major planets must contain large quantities of
liquid or solid hydrogen and helium.

This conclusion is in full accord with expectation.
The planets are believed to have been formed in
some way from the Sun, so that initially their com-

138 LIFE ON OTHER WORLDS

position must have been generally similar to that of
the outer layers of the Sun. Now we have learnt in
recent years a good deal about the composition of the
outer layers of the Sun. The principal constituent
is hydrogen; the Sun seems, in fact, to consist of
hydrogen to the extent of about one-third part by
weight. Next in abundance to hydrogen come
helium, oxygen and carbon, followed by nitrogen,
silicon and the metallic elements. As massive
planets, like the four major planets, are able to
retain the light constituents of their original atmo-
spheres, it is to be expected that large amounts of
hydrogen and helium are contained in their atmo-
spheres at the present time. There is no means,
however, by which this conclusion can be tested by
observation.

The spectra of the major planets are of great
interest and give some partial information about the
composition of their outer atmospheres. In the
early days of the application of the spectroscope to
astronomy, Huggins discovered, by visual examina-
tion of the spectrum of Jupiter, that there was a
strong absorption band in the orange region of the
spectrum and that there were several weaker bands
in the green region. These bands appear more
strongly in the spectrum of Saturn than in that of
Jupiter, but they are not present in the spectrum of
the rings of Saturn; this provides conclusive proof
that they originate in the atmosphere of Saturn.
Uranus and* Neptune show for the most part the
same bands, but with still greater intensity, together
with some additional ones. As we proceed from
Jupiter outwards to Neptune we accordingly find

THE GIANT PLANETS 139

that there is a great increase in the selective absorp-
tion of light in the yellow, red and infra-red regions
of the spectrum. For Uranus and Neptune the
absorptions are so strong that most of the yellow and
red regions of their spectra are lost by the absorp-
tion; it is because of this loss of the light of long
wave-length that these two planets appear green
when seen in the telescope. More recent investiga-
tions by Dr. Slipher, at the Flagstaff Observatory,
Arizona, have extended the spectra of the major
planets to longer wave-lengths far into the infra-red
region and have revealed several more strong
absorptions in that region.

When these absorptions in the spectra of the
major planets were discovered, and for more than
sixty years afterwards, it was not known what their
origins were. They had never been observed in
any spectra in the laboratory, and for many years
they remained one of the unsolved problems of
spectroscopy. The clue to their identification first
came in the year 1932 from purely theoretical
considerations. From the theoretical study of
molecular spectra Dr. Wildt came to the conclusion
that some of the absorptions agreed in wave-length
with bands that should be present in the spectrum of
ammonia and that others agreed in wave-length
with bands that should be present in the spectrum of
methane or marsh-gas, the gas that is given off by
decaying vegetation, and is known to the coal- miner
as the deadly fire-damp. These conclusions, based
on theoretical reasoning, were subsequently con-
firmed by laboratory observations; the reason why
the bands had not been detected in the laboratory

140 LIFE ON OTHER WORLDS

before theoretical reasoning had suggested their
identification was that a considerable quantity of
gas is required for their production with sufficient
intensity to be easily observed.

After the origin of the absorptions had been con-
firmed in the laboratory and the absorptions
produced by ammonia and marsh-gas had been
fully investigated, the spectra of the major planets
were photographed by Dr. Dunham, using the 100-
inch telescope at the Mount Wilson Observatory,
with optical power far superior to that which had
been available to Dr. Slipher. The more detailed
investigation that was possible with the aid of this
giant telescope showed that there was a complete
and detailed coincidence between the spectra of
ammonia and marsh-gas obtained in the laboratory
and the absorptions shown in the spectra of the
major planets.

A comparison between the strength of the absorp-
tions in the spectrum of Jupiter and the strength of
the absorptions produced by passing light through
a tube of known length containing ammonia gas
at atmospheric pressure, enabled Dr. Dunham to
conclude that the quantity of ammonia gas pro-
ducing the absorptions in the spectrum of Jupiter is
equivalent to a layer 30 feet thick under standard
conditions of temperature and pressure. The
amount of ammonia in the atmosphere of Saturn is
not so great, the ammonia absorptions being weaker
in the spectrum of Saturn than in ; that of Jupiter.
Ammonia has not been detected at all in the spectra
of Uranus and Neptune.

In the spectra of all the four major planets the

THE GIANT PLANETS 14!

absorptions produced by marsh-gas are much
stronger than those produced by ammonia and, as
we have already mentioned, Uranus and Neptune
appear green because the yellow and red regions of
their spectra are to a large extent cut out by the
very great intensity of these absorptions. It was
found by Drs. Adel and Slipher in 1935, that a
column of marsh-gas, forty-five feet in length, and at
a pressure of forty atmospheres, gave absorption
bands that were intermediate in intensity between
those of the bands present in the spectra of Jupiter
and Saturn. The much greater strength of the
absorptions due to marsh-gas in the spectra of the
two more remote planets, Uranus and Neptune, is
probably accounted for by the lower temperature
of these planets. At the very low temperatures that
prevail on Uranus and Neptune, all the ammonia
is frozen out of their atmospheres ; this explains why
no ammonia can be detected in the spectra of
Uranus and Neptune. The absence from the atmo-
spheres of these two planets of clouds consisting of
droplets of liquid ammonia or small crystals of
frozen ammonia, which must be present in the
atmospheres of Jupiter and Saturn, makes it possible
to see through the atmospheres of Uranus and
Neptune to a much greater depth than in the case of
Jupiter and Saturn. Adel arid Slipher have
estimated that twenty-five miles of marsh-gas at
atmospheric pressure would be required to ^ive
absorptions as strong as those that are present in
the spectrum of Neptune. When it is recalled that
the equivalent thickness of the atmosphere of the
Earth at atmospheric pressure is only 5^ miles, we

142 LIFE ON OTHER WORLDS

have here very direct and conclusive evidence of an
atmosphere far more extensive than that of our
Earth.

It might be expected that, marsh-gas being such
an important constituent of the atmospheres of the
major planets, other gaseous hydrocarbons would
also be present in their atmospheres. Such sub-
stances as ethane, ethylene and acetylene have
been looked for in vain. Ammonia and marsh-gas
between them account, in fact, for all the absorp-
tions detected in the spectra of the major planets
and there are no absorptions remaining to be
accounted for by other gaseous constituents. It is
a grand slam.

The gaseous atmospheres of the major planets,
which form the upper regions of their outer low-
density layers, are, therefore, entirely different from
the atmosphere of the Earth. Hydrogen and
helium must be present in these atmospheres in large
quantity and the other inert gases, argon, krypton,
etc., must also be present. There is a considerable
amount of the poisonous marsh-gas and, in the case
of Jupiter and Saturn, some of the pungent am-
monia gas also; the temperatures are too low for
ammonia to be present in large amounts. There
cannot be any carbon dioxide, because of the low
temperature; there is unlikely to be any free
nitrogen and there will certainly be no free oxygen.

How does it came about that we find on these
planets atmospheres that contrast so strangely with
the atmosphere with which we are familiar on the
Earth ? Can we explain how such atmospheres
have come into existence ? In a general way we

THE GIANT PLANETS 143

are able to provide a satisfactory explanation and to
show that the surprising difference between the
atmospheres of the great planets and the atmosphere
of the Earth is the result of the great planets having
been able to retain all their hydrogen, whereas most
of the hydrogen that was present in the initial
atmosphere of the Earth was able to escape whilst
the Earth was still hot.

Let us consider what is likely to have been the
course of events whilst one of these giant planets
cooled. With the gradual fall in temperature from
its initial high value, a stage would be reached when
liquefaction would set in, giving rise to a liquid core
surrounded by an extensive gaseous atmosphere.
In the presence of hydrogen at a high temperature,
the oxides of iron are reduced, metallic iron being
produced. Most of the iron would, therefore, go
into the liquid core in its metallic state, oxygen being
set free and entering into the atmosphere. The
oxides of the remaining principal constituents of the
rocks, including potassium, sodium, magnesium,
calcium and silicon, are not reduced in the presence
of hydrogen at high temperature. They would,
therefore, combine to form liquid rock masses,
generally similar in composition to terrestrial rocks.
With still further cooling, the rock crust would
begin to solidify; there would still be a liquid core of
molten iron and the atmosphere at this stage would
consist of hydrogen, helium, oxygen, nitrogen,
carbon and smaller quantities of the inert gases,
as well as of sulphur, chlorine and other elements.

As the planet cooled and the temperature fell still
further, chemical action would take place. The

PLATE 10

SPECTRA OF THE PLANETS

The plate shows portions of the spectra of Venus,
Mars, Jupiter and Saturn, to illustrate the composition
of their atmospheres.

The first strip shows corresponding portions of the
spectra of the Sun and Venus on the same scale. Many
of the absorption lines are common to the two spectra ;
these absorptions are produced in the outer layers of
the Sun. A number of additional strong absorptions
are to be seen in the spectrum of Venus, which are not
present in the spectrum of the Sun. These are all pro-
duced by carbon dioxide in the atmosphere of Venus.

The second strip shows portions of the spectra of Mars
(above) and the Moon (below) taken when at the same
altitude under conditions of exceptional atmospheric
dryness by Dr. Slipher at the Flagstaff Observatory
(altitude 7,180 feet). The absorption due to water-
vapour (marked a) is stronger in the spectrum of Mars
than in that of the Moon, indicating the presence of
water-vapour in the atmosphere of Mars.

The third strip shows the spectrum of the ball of
Saturn (centre) and of the rings (above and below).
Between D and C, and to the right of B, absorptions are
seen in the spectrum of Saturn which are absent from
the spectrum of the rings. These are caused by marsh-
gas in the atmosphere of Saturn.

The bottom strip shows corresponding portions of
the spectra of the Sun, Saturn and Jupiter. Absorp-
tions produced by ammonia gas (snown under the
spectrum of Jupiter) are present in the spectrum of Jupiter
and, less strongly, in the spectrum of Saturn, but are
absent from the Sun's spectrum. This proves that
ammonia is present in the atmospheres of Jupiter and
Saturn.

' - &%%ftW$ ' f'

e g ,,7W

' . tid-

* '*;,:

PLATE 1 1

CLOUDS ON THE PLANET VENUS

A portion of a series of photographs of Venus in ultra-
violet light, taken by Dr. F. E. Ross in June 1927 with
the Go-inch reflector of the Mount Wilson Observatory,
is reproduced. The contrast on the original negatives
has been greatly increased by copying in order to make
the markings more readily visible. Corresponding
markings are not shown on photographs in red light.

A comparison of photographs taken on the same day
shows the reality of the recorded markings. These are
of two types : bright clouds, which appear as bulges at
the limb, and dark clouds, which appear as depressions
at the limb. The dark clouds are most frequently seen
near the " terminator," the dividing line between the
sunlit and dark parts of the planet.

The bright clouds are probably thin cirrus clouds
above the permanent cloud layer, overlying a yellowish
atmosphere. Where this upper atmosphere is free from
cloud, no ultra-violet light is reflected back and the
appearance is that of a dark cloud.

146 LIFE ON OTHER WORLDS

carbon would combine with some of the oxygen to
form carbon dioxide. The remainder of the oxygen
would combine with some of the hydrogen, which,
it may be recalled, was by far the predominating
constituent of the atmosphere, to form steam. The
atmosphere would then consist largely of hydrogen,
helium and the other inert gases, carbon dioxide
and steam. By this time the temperature would
have fallen to somewhere about 1,000 C.

Now carbon dioxide can combine under suitable
conditions with hydrogen to form marsh-gas and
water-vapour. But the reaction is what chemists
term a reversible one. Under other circumstances
it can proceed in the reverse direction, so that the
marsh-gas then reacts with water-vapour to give
carbon dioxide and free hydrogen. The formation
of marsh-gas is accompanied by a decrease in
volume and is therefore favoured by high pressure,
whereas high temperature tends to cause the reac-
tion to proceed in the reverse direction. At the
temperature of 1,000 C. and the relatively low
pressure which then prevailed on the planet, the
predominant tendency is for the mixture of carbon
dioxide and hydrogen to be stable; but as the tem-
perature fell there would be an increasing amount
of marsh-gas formed. When the temperature had
fallen below about 300 C. practically all the carbon
dioxide would have been converted into marsh-gas.
The presence of marsh-gas or methane in the
atmosphere of a giant planet might, therefore, have
been anticipated.

We have mentioned that no traces can be de-
tected in the atmospheres of the major planets of

THE GIANT PLANETS 147

other hydrocarbons such as acetylene, ethane,
ethylene and so forth. But it is possible for these
hydrocarbons also to be formed from carbon
dioxide and hydrogen. How, then, are we to
account for their absence ? The explanation lies in
the facts that molecules of these substances are
readily broken up by the action of ultra-violet light,
and that they are also attacked by hydrogen atoms,
the carbon chains being broken up. As a result of
such processes the higher hydrocarbons would be
completely destroyed in a time that is short com-
pared with the age of the planets, methane being
the end result of this process. The molecules of
methane are themselves broken up by the action of
ultra-violet light, but in an atmosphere containing
free hydrogen the methane is again formed. When
there is an ample supply of hydrogen, as there cer-
tainly is in the atmospheres of the giant planets, a
steady state results in which, though molecules of
methane are being continually broken up and
formed again, there is a constant amount of methane
present in the atmosphere.

It has been suggested that the formation of methane
and steam from carbon dioxide and hydrogen at
a temperature of about 300 C. would be greatly
facilitated by suitable activation. The partially
reduced oxides of iron, which should be present on
the rocky surface of the planet exposed to hot
hydrogen, would provide a suitable agent for such
activation and would ensure that all the carbon
dioxide would rapidly be transformed.

As the temperature fell still further there would
be combination between the nitrogen in the atmo-

148 LIFE ON OTHER WORLDS

sphere and some of the hydrogen to produce
ammonia. This, again, is a reversible reaction and
the ammonia will in turn break down into nitrogen
and hydrogen. The lower the temperature and the
higher the pressure, the greater will be the concen-
tration of ammonia. It has been shown that the
presence of methane will tend to prevent the de-
composition of ammonia. The net result is that as
the temperature falls below about 200 C. the pro-
portion of free nitrogen to ammonia will decrease
rapidly.

The planet continued to cool, and at length the
water-vapour began to condense and ultimately an
ocean of very great depth was formed. In con-
densing, the water vapour would carry ammonia
gas with it in solution, so that the ocean would be
strongly alkaline with ammonia. Compounds of
sulphur and chlorine with hydrogen, such as the
evil-smelling sulphuretted hydrogen, which may
have been present in the atmosphere in small
amounts, would also be carried down in solution.
With a still further fall in temperature a stage at
length arrived when the oceans froze. It may be
noted that a solution containing one part of
ammonia in two of water freezes at 100 C.; the
major planets are all colder than this.

The final result, after the planet had cooled to
about its present temperature, was an atmosphere in
which hydrogen is the predominant constituent, with
helium present in considerable amount, and argon,
neon and the other rare gases present in lesser pro-
portions. There would be no water-vapour, for it
had all been frozen out, no carbon dioxide and no

THE GIANT PLANETS 149

nitrogen. Methane should be present in consider-
able amount, but ammonia in moderate amount
only, for it had mostly been carried down in solu-
tion. In planets as cold as Uranus and Neptune
the residual ammonia would be precipitated as a
solid ; even in the atmosphere of Jupiter it is not far
from the point of precipitation.

This fact may be used to obtain an estimate of
the minimum possible temperature of the visible
surface of Jupiter. The amount of ammonia in
the atmosphere of Jupiter is estimated to be equiva-
lent to a layer thirty feet thick under standard con-
ditions. Below a certain temperature it would not
be possible for the quantity of ammonia, which is
observed to be present in the atmosphere of Jupiter,
to exist in the atmosphere; it would be partially pre-
cipitated by its own weight. It can be shown from
theoretical considerations that in an atmosphere
consisting mainly of hydrogen, the lowest tempera-
ture possible at which the observed quantity of
ammonia could be present is 120 C. This is in
close agreement with the temperature determined
by direct observation. The amount of ammonia in
the atmosphere of Saturn is less than in that of
Jupiter and is consistent with a temperature some
15 C. lower; this, again, is in close accordance
with the observed temperature.

From purely theoretical reasoning, therefore, we
should expect to find atmospheres on the giant
planets which are closely in accordance with what
observation reveals. The crucial fact involved in
their interpretation is the presence of a large excess
of hydrogen; it is to this abundance of hydrogen

150 LIFE ON OTHER WORLDS

that the marked contrast between the atmospheres
of these planets and the atmosphere of the Earth is
due. In their broad outlines the main facts about
the constitution of these atmospheres are explained.
The general line of reasoning will apply to all very
massive planets. We may expect them to have
very extensive atmospheres, containing hydrogen,
helium and marsh-gas; but no oxygen.

There still remain many points of detail to be
explained, such as the nature of the belts on Jupiter
and Saturn, the nature of the Red Spot and South
Tropical Disturbance on Jupiter and the variety of
colours, ranging from white through pink and
brown, which are to be seen upon the surface of
Jupiter. It has been suggested that these colours
may be due to the formation of small quantities of
iridescent compounds involving sodium. It is
known that sodium is present in small quantity in
the atmosphere of the Earth at very great heights
and the suggestion has been made that the sodium
has been swept up by the Earth as it moves with
the Sun through space, for sodium is known to be
present in very small amount in interstellar space.
If this were so and it must not be regarded as
having been definitely established the presence of
sodium in the outer atmosphere of Jupiter could be
accounted for in the same way. This attempt to
account for the colours to be seen on Jupiter can
scarcely be regarded at present as much more than
mere speculation, but it is mentioned here as an
indication of the lines along which attempts are
being made to explain some of the phenomena
that are still very puzzling. A possible explanation

THE GIANT PLANETS 15!

of the Red Spot has recently been suggested by
Wildt. He considers that it may be a vast solid
body floating in an ocean of permanent gases.
The nature of the solid body is not known, but we
may think of it as analogous to an iceberg floating
in the ocean; it may consist of solid hydrogen.
Peek has pointed out that if the level at which it
floats is subject to slight variations, the changes in
rotational velocity of the spot can be accounted
for. In this connection he remarks that whenever
the spot becomes conspicuously dark the rotational
period lengthens. The changes of colour may be
associated with changes of level. It seems that in
this way it may be possible to explain the principal
phenomena of the Red Spot.

The giant planets are worlds in strange contrast
to our own with their enormously deep coatings of
ammoniated ice, covered to a depth of thousands
of miles with solid or liquid gases, over which are
atmospheres devoid of oxygen or water-vapour but
containing large quantities of poisonous marsh-gas.
These dreary, remote, frozen wastes of the solar
system are not worlds where we can hope to find
life of any sort. Great cold may not by itself make
life impossible, even though it may make its de-
velopment extremely improbable; nor by itself may
great pressure. But when these conditions are com-
bined with absence of oxygen and of moisture and
with abundance of poisonous gases, we have such a
combination of unfavourable circumstances that
we are compelled to turn elsewhere in our quest for
life in the universe.

CHAPTER VII

VENUS THE EARTH'S TWIN
SISTER

OF all the planets in the solar system, Venus most
nearly resembles the Earth in size, in mass and in
mean density : and so it is on Venus that we have the
greatest expectation of finding conditions akin to
those that exist on the Earth.

The path of Venus lies inside the path of the
Earth. When Venus is at its nearest to the Earth
it is only 26 million miles away. No other body
ever comes so near the Earth, with the exception of
the Moon and an occasional comet or asteroid.
When at its farthest from the Earth, Venus is 160
million miles away. With such a wide range
between its greatest and least distances, it is natural
that at some times Venus appears much brighter
than at others. When at its brightest it is easily
seen with the naked eye in broad daylight. When
I lived in South Africa, I was sitting by the sea one
day when, by chance looking upwards, I saw, as I
thought, an aeroplane flying at a great height,
glittering brightly in the sunshine against the azure
blue sky. But as I watched, I noticed that it
remained in the same position, and only then
realised that I was looking, not at an aeroplane,
but at the planet Venus.

When the orbit of a planet lies outside the orbit
of the Earth the planet is, at times, visible through-
out the night. When, however, the orbit is within

152

VENUS THE EARTH'S TWIN SISTER 153

the orbit of the Earth, the planet can never be seen
at midnight. It is visible either in the evening after
sunset or in the morning before sunrise. Venus,
because of its brightness, becomes a conspicuous
object in the twilight sky and is often referred to as
the evening star or the morning star according as it
is seen after dusk or before dawn. The Greeks had
two names for it; they called it Phosphorus when
it appeared at dawn and Hesperus when it appeared
at dusk. The identity of the evening and morning
star was known, nevertheless, as early as the time
of Pythagoras.

Suppose we observe Venus night after night with
a telescope. Commencing when it first appears
low in the western sky just after sunset, we find
that it shows a full sun-illumined disk; as it draws
gradually away from the Sun night after night for
nearly eight months it appears progressively brighter
and larger, but the portion of the disk that is visible
becomes gradually less. When at its greatest
elongation from the Sun, it shows the half-moon
phase. Then it will be seen to turn back, drawing
gradually nearer to the Sun and developing a
crescent phase that becomes narrower and narrower;
the diameter of the image continues to increase; and
for a time the planet continues to become brighter.
The greatest brightness is reached about 32 days
after the greatest eastern elongation, the phase then
corresponding to that of the Moon when five days
old. About ten weeks after eastern elongation,
Venus vanishes in the Sun's rays; greatly diminished
in . brightness, it then shows a large but narrow
crescent. A little later it appears as the morning

154 LIFE ON OTHER WORLDS

star and corresponding changes are passed through
in the reverse order.

It is of interest to recall that the phases of Venus
were first discovered in 16.10 by Galileo, who was
able to see the changing phases in his telescope.
As was frequently done at that time, he announced
his discovery in the form of an anagram :

Haec immatura a me iamfrustra leguntur : o.y.

The solution of the anagram, which concealed his
discovery, was given some months later as:

Cynthia figuras amulatur Mater Amorum,

which means that " The Mother of the Loves (i.e.
Venus) imitates the phases of Cynthia (i.e. the
Moon)."

Venus has been called the twin sister of the Earth
because it closely resembles the Earth in size and
weight. It is a little smaller than the Earth, its
diameter being 7,700 miles as compared with the
Earth's 7,927 miles. The area of its surface is
therefore five per cent, smaller than the area of the
surface of the Earth. Its weight is about four-fifths
of the weight of the Earth. The velocity of escape
from Venus is only slightly less than the velocity of
escape from the Earth and it is therefore to be
anticipated that Venus will be found to possess a
fairly extensive atmosphere, comparable in extent
with the atmosphere of the Earth.

The presence of an atmosphere on Venus is
readily proved by observation. When Venus is
between us and the Sun, showing the narrow
crescent phase, the tips of the horns of the crescent

VENUS THE EARTH'S TWIN SISTER 155

are not at the two ends of a diameter as they are
in the case of the crescent Moon, but the horns
extend well round the circumference of the dark
limb. This means that there is a region of twilight
on Venus; the Sun is not shining directly on this
portion of the planet but it becomes faintly visible
by the light scattered in the atmosphere. If the
Earth had no atmosphere, darkness would come
suddenly as soon as the Sun had set; the scattering
of the sunlight by the Earth's atmosphere for some
time after the Sun has disappeared below the
horizon causes the gradual transition from day to
night, which we term twilight.

Direct evidence of an atmosphere on Venus is
also obtained at times of the transits of Venus. I
have mentioned that the path of Venus lies inside
the path of the Earth and it must sometimes happen
that Venus will pass directly between us and the
Sun. We then see Venus as a dark spot moving
across the face of the Sun. If the orbits of Venus
and the Earth were in the same plane, Venus
would pass in front of the Sun every time that it
changed from an evening to a morning star. The
orbit of Venus is, however, inclined to the orbit of
the Earth as an angle of about 3^; Venus can
therefore only be seen projected on the disk of the
Sun at a time when it is close to one of the two
points where its orbit crosses the plane of the orbit
of the Earth. Transits of Venus across the Sun
are somewhat rare phenomena. The last two
transits occurred on December 8th, 1874, and
December 6th, 1882; the next two transits will
occur on June yth, 2004 and June 5th, 2012. When

156 LIFE ON OTHER WORLDS

a transit of Venus takes place and Venus is just
entering upon the disk of the Sun or just leaving it
the edge of the portion of Venus that is outside the
Sun is surrounded by a bright line of light. This
appearance can only be caused by the scattering of
sunlight by an atmosphere around Venus. It will
be remembered that this appearance is not seen
when Mercury transits on to the Sun's disk, because
Mercury has no atmosphere.

When we look at Venus in the telescope, we are
apt to be disappointed. As we have already
mentioned, she shows phases like the Moon,
depending upon the proportion of her sunlit face that
is turned towards us. We might hope to find some
evidence of continents and oceans on a world that
is in some respects so similar to our own. But we
are doomed to disappointment. There are no
well-defined markings to be seen on her surface.
Nothing more than faint ill-defined shadings may
be seen, and these can only be seen occasionally.
They are very elusive, for there is so little contrast
between the markings and the rest of the disk that
they are barely visible. They have been described
as " large dusky spots " and are certainly not
permanent. Such markings as are from time to
time visible are therefore certainly not surface
markings; they must be produced in the planet's
atmosphere.

Venus must either be covered with a permanent
layer of cloud or her atmosphere must be so hazy
that light from the Sun cannot penetrate to the
surface and out again. The light that reaches us
from Venus must either have been reflected from

VENUS THE EARTH'S TWIN SISTER 157

a layer of cloud or have been scattered within the
atmosphere without penetrating to the surface.

It might be hoped that some further information
could be obtained by photographing Venus on
plates sensitive to the long wave-length infra-red
light. Within recent years there has been a great
development in photography in the use of infra-red
or haze-cutting plates. Everybody is familiar with
the way in which, on a fine day, with a slight haze,
the detail in a distant landscape is lost. But if,
under such conditions., we take a photograph using
a plate that has been specially sensitised for the
infra-red light, much detail that is invisible to the
eye will be clearly shown. This is illustrated in
Plate 1 2 where two photographs are reproduced,
showing the view from the top of Mount Hamilton
in California, where the Lick Observatory is located,
looking across the valley to the distant mountains.
One photograph was taken with a plate sensitive
to the short-wave ultra-violet light, the other with
a plate sensitive to the long-wave infra-red light,
the two photographs being taken at the same time
and therefore under the same conditions. The
ultra-violet photograph dirnly shows the skyline of
the distant mountains but none of the intervening
detail, which is clearly revealed by the infra-red
photograph.

We therefore photograph Venus using these infra-
red plates, in the hope that in this way something
will be revealed that the eye cannot detect. Venus,
however, will not be circumvented in this way.
She refuses to reveal her secrets; the plate shows
us no more than our eyes can see. The infra-red

158 LIFE ON OTHER WORLDS

light is no more successful in penetrating to the
surface and out again than the ultra-violet light
had been.

There is one difference that may be noted between
the photographs in ultra-violet light and those in
infra-red light. The former record bright mark-
ings, which rapidly change their form &nd are of
short duration. Photographs of Venus in ultra-
violet light, showing some of these bright markings
and illustrating the rapidity with which they
change, are reproduced in Plate 1 1 . These markings
are not seen on the photographs in the infra-red
light. They indicate the existence of some sort of
atmospheric haze or possibly of very thin clouds
at a high altitude, through which the light of
long wave-length can pass without difficulty,
but which scatters or reflects the light of short
wave-length. - ,

Venus reflects about 60 per cent, of the sunlight
that falls upon it. This high reflecting power,
which contrasts with the low reflecting powers
about seven per cent. of both Mercury and the
Moon, is very much what we should expect from a
planet covered with a thick layer of clouds. The
visible surface is practically white almost devoid
of colour; this again is consistent with the appear-
ance of a cloud layer. An estimation of the height
to which the atmosphere of Venus extends can be
made from the amount of the prolongation of the
horns, when the planet is seen as a narrow crescent.
It can be shown that the portion of the atmosphere
where the twilight is sufficiently bright to be seen
through the glare of our own atmosphere extends

VENUS THE EARTH'S TWIN SISTER 159

to a height of about 4,000 feet above the visible
surface of Venus. The total height of the atmo-
sphere of Venus must, of course, be many times
greater than this, for the faint twilight effect pro-
duced by the more tenuous upper reaches of the
planet's atmosphere would be lost in the glare of
our own atmosphere. It seems probable, never-
theless, that the atmosphere of Venus above the
visible surface is both less extensive and less dense
than the atmosphere of the Earth. We can more
fairly compare it with the portion of the Earth's
atmosphere above the high clouds. This seems to
suggest that these observations by no means reveal
the full extent of the atmosphere of Venus, which
we should expect to be fairly comparable with that
of the Earth. It therefore seems reasonable to
conclude that the visible surface is a permanent
layer of cloud, which we have no means of pene-
trating, situated at a fairly high level above the
surface of the planet.

The ill-defined nature of the markings seen on
Venus and their impermanence make it difficult
to determine the length of the day on Venus with
any accuracy. The conclusions reached by dif-
ferent observers have been very varied and some-
what contradictory. Some have asserted that the
length of the day on Venus is about the same as the
length of the day on the Earth; others have con-
cluded that Venus always turns the same face to
the Sun, just as Mercury does, so that her day is
equal to her year, which is 225 of our days. Most
probably the truth lies between these two extremes.
It was through the agency of the tides raised by the

PLATE 12

PHOTOGRAPHS OF MARS AND OF TERRESTRIAL

LANDSCAPE

The plate shows photographs of Mars (a and b} and
of San Jos6, taken from the top of Mount Hamilton,
California (c and d). The photographs a and c were
taken with ultra-violet light (short wave-length) , b and d
were taken with infra-red light (long wave-length) . The
distant mountains and the town of San Jose in the valley
are closely seen in d but are obliterated in c. The dis-
tance of San Jose from the point where the photograph
was taken is 13 J miles; the short wave-length ultra-violet
"light was unable to penetrate through this extent of the
Earth's atmosphere. The surface markings of Mars are
clearly showrji in b, but not in a, indicating the presence
on Mars of an atmosphere of sufficient extent to prevent
the ultra-violet light from penetrating to the surface of
Mars and out again.

Photographs by Dr. W. H. Wright, Lick Observatory,
with Grossley Reflector.

PLATE 13

CLOUDS ON THE PLANET MARS

The series of six photographs, taken in ultra-violet
light, show the formation and growth of a white cloud
during the course of a Martian afternoon. The photo-
graphs, in order from left to right, were secured at
intervals through a period of about four hours, during
which the planet rotated 55. The place of the brightest
part of the cloud, in the successive positions of the planet,
is shown by the arrows in the lower row, the rotational
movement being upwards and to the left. The cloud is
not visible in the first picture, but becomes so in the
second and increases in strength as it is carried through
the Martian afternoon to sunset. Photographs taken on
1926, October 16, at the Lick and Mount Wilson
Observatories .

The lower series of photographs shows the formation
of a yellow cloud, visible in infra-red but not in ultra-
violet light. The centre photograph is taken in ultra-
violet light, the others in infra-red light, the markings
being displaced between these two photographs by the
rotation of Mars. If the photographs are turned 90,
the markings will be seen to have the appearance of a
stag's head. In the left-hand photograph will be seen,
just under the stag's neck, a bright patch, which is not
visible on the right-hand photograph, nor in the photo-
graph in ultra-violet light. This bright patch is a
yellow cloud. Photographs by Dr. Wright, 1926,
November 23, at the Lick Observatory.

l62 LIFE ON OTHER WORLDS

action of the Sun on Mercury before it solidified
that the rotation of Mercury was slowed down until
it eventually turned the same face always to the
Sun; similarly, the tides raised on the Moon by the
attraction of the Earth slowed down the rotation
of the Moon until it always turns the same face to
the Earth. The tides raised on Venus by the
gravitational attraction of the Sun would have been
much smaller than those raised on Mercury, because
Venus is at a considerably greater distance. The
tide-raising force falls off inversely as the cube of
the distance, and at the distance of Venus it is only
about one-sixth as great as at the distance of
Mercury. It is consequently to be anticipated
that, though the rotation of Venus would be slowed
down to a moderate extent by the friction of the
tides raised on her by the Sun, the effect would not
be nearly large enough to cause her always to turn
the same face to the sun.

That the rotation period of Venus is not as short
as a day is quite certain. If her rotation were as
rapid as this, there would be no difficulty in detect-
ing it by comparing the spectra of the light from
the east and west limbs. The relative motion of
the two limbs, one moving towards the Earth and
the other away, would produce small relative shifts
between the wave-lengths of the corresponding lines
in the two spectra, which would be readily observed.
No such shift has been detected. The inference
from this failure to detect a relative shift is that the
period of rotation rhust be at least several weeks in
length. The same conclusion has been reached
recently by M. Antoniadi from observations of the

VENUS THE EARTH'S TWIN SISTER 163

faint markings seen on Venus with the aid of the
great telescope at Meudon, near Paris.

It seems reasonably certain, on the other hand,
that Venus does not turn the same face always to
the Sun. The temperature of the sunlit side of
Venus has been found by observation to be about
50 to 60 C., whilst that of the dark side is about
20 C. A much lower temperature of the dark
side would be expected if it never received any heat
from the Sun, but received heat only through con-
vective motions of the atmosphere. The sunlit face,
on the other hand, would be very much hotter than
it is observed to be if it were continuously receiving
heat from the Sun. The difference between the day
and night temperatures on Venus is, therefore,
considerably smaller than it would be if Venus
always turned the same face to the Sun. We shall
probably not be far wrong if we assume that the
length of the day on Venus is equal to about four or
five of our weeks. The year on Venus being equal
to 225 of our days, it follows that on Venus there are
only some six or seven days in the year.

The measurements of the temperature on Venus
do not refer to the true surface of the planet nor to
what we may term the visible surface. The tem-
perature of the true surface of the planet, beneath
the permanent layer of cloud, is almost certainly
appreciably higher than the temperature found
by observation. There must be a considerable
green-house effect beneath the^ cloud layer, the
short-wave radiation from the Sun being absorbed
and given out again as long wave-length heat
radiation. It is quite possible that at the surface of

164 LIFE ON OTHER WORLDS

Venus, in the equatorial regions, the temperature
may be as high as, or even higher than, that of
boiling water.

The light reflected from Venus to the Earth has
been analysed to find out whether it shows the
absorptions that are characteristic of oxygen or
water-vapour. Neither oxygen nor water-vapour
has been detected. This negative result does not
necessarily imply that there is neither oxygen nor
water-vapour in the atmosphere of Venus, but
merely that the amount is not sufficient to be
revealed by the tests that can be used. The tests
for one substance may be extremely sensitive and
the presence of a very small quantity can then be
detected, whilst the tests for another substance may
be so insensitive that it must be present in very great
abundance before there can be any hope of detect-
ing it. The most striking illustration of differ-
ences in the sensitivity of the tests for the presence of
different substances is provided by the comparison
between calcium-vapour and hydrogen in the outer
layers of the Sun. The calcium-vapour gives rise to
two extremely strong absorptions in the spectrum of
sunlight, absorptions which are by far the strongest
in the whole range of the Sun's spectrum that we
are able to study. These intensely strong absorp-
tions are produced by an amount of calcium-vapour
which, under standard conditions in the laboratory,
would be less than half an inch in thickness. The
absorptions produced by hydrogen are weak rela-
tively to those produced by calcium, yet the
hydrogen is so abundant that it is estimated that,
atom for atom, hydrogen is at least 300 times as

VENUS THE EARTH'S TWIN SISTER 165

abundant as the whole of the metallic vapours
together.

The tests for the presence of water-vapour in the
atmosphere of Venus are less sensitive than those for
the presence of oxygen. An amount of oxygen on
Venus equal to a thousandth part of that above an
equal area of the earth could certainly have been
detected. It must be remembered, however, that
we are unable to see down to the surface of Venus
and any tests that we can apply give information
only about the portion of the atmosphere that is
above the permanent layer of cloud. Nevertheless,
if the oxygen above the cloud layer were as much
as the one-hundredth part of the oxygen in the
atmosphere of the Earth above the highest clouds, it
would have been detected. There is no escape
from the conclusion that there can be very little, if
any, oxygen in the atmosphere of Venus.

The failure to detect water-vapour, even though
the tests are less delicate than those for oxygen, may
appear at first sight to be rather surprising. For if
Venus is covered with a permanent layer of clouds
there must be water-vapour, unless the clouds are
of a different nature from those that occur in our
own atmosphere. No alternative suggestion, which
is at all feasible, of the nature of the clouds on Venus
has been made and it seems certain that they must
consist of water droplets, similar to the clouds in the
Earth's atmosphere. The explanation of the appar-
ent absence of water- vapour must be that the atmo-
sphere above the clouds is extremely dry. Most of
the water-vapour has been condensed out of the
upper atmosphere and what is left is not sufficient

l66 LIFE ON OTHER WORLDS

for us to be able to detect. In this respect the
atmosphere of Venus seems to be similar to that of
the Earth. In the atmosphere of the Earth the
water- vapour is confined almost wholly to the lower
layers and the amount above a height of five miles is
always small. If the cloud layer on Venus reaches
to a height of four or five miles above the surface,
the failure to detect water-vapour in its atmosphere
presents no difficulties.

The most interesting and significant fact about
the atmosphere of Venus is the great abundance of
carbon dioxide. In 1932, Adams and Dunham,
using the great loo-inch telescope at the Mount
Wilson Observatory, discovered three strong absorp-
tions in the spectrum of Venus in the long wave-
length infra-red region. These absorptions are not
found in the spectrum of the Sun, even when setting.
They are not produced, therefore, in the atmosphere
of the Earth but must originate in the atmosphere of
Venus. At the time that they were discovered they
had not been observed in any terrestrial spectrum
and it was not known, therefore, what substance
produced them. Theoretical investigations sug-
gested that they might be due to carbon dioxide.
This was confirmed when Dunham succeeded in
obtaining a faint absorption, corresponding in
position with the strongest of the three absorptions,
by passing light through forty metres of carbon
dioxide at a pressure of ten atmospheres. Later,
Adel and Slipher were able to obtain all three
absorptions by passing light through forty-five
metres of carbon dioxide at a pressure of forty-seven
atmospheres. The three absorptions coincided

VENUS THE EARTH'S TWIN SISTER 167

exactly in position with those observed in the spec-
trum of Venus but, even with so great a thickness
of carbon dioxide, they were less intense than the
corresponding absorptions in the spectrum of
Venus.

Their laboratory investigations enabled Adel and
Slipher to conclude that the amount of carbon
dioxide above the visible surface of Venus is equi-
valent to a layer two miles in thickness at standard
atmospheric pressure and temperature. This
affords direct experimental confirmation of the
conclusion, which we have already reached, that
Venus has an extensive atmosphere. The interest
of the result lies, however, in the comparison be-
tween the amount of carbon dioxide on Venus and
the amount in the Earth's atmosphere. The
amount of carbon dioxide present in the path of
sunlight, when the Sun is setting, is equivalent to a
thickness of about thirty feet only. Carbon dioxide
is accordingly vastly more abundant in the atmo-
sphere of Venus than in that of the Earth.

The amount of carbon dioxide observed to be
present in the atmosphere of Venus, equivalent to a
layer two miles in thickness at standard atmospheric
pressure and temperature, represents only the
portion of the carbon dioxide that is above the
permanent cloud layer the visible surface. The
total amount above the solid surface of the planet
may well be appreciably greater. When we recall
that the whole atmosphere of the Earth at standard
pressure and temperature would form a layer only
about 5! miles in thickness and that we should
expect the atmosphere of Venus to be rather less

l68 LIFE ON OTHER WORLDS

extensive than that of the Earth, we are forced to
the conclusion that carbon dioxide is an important
constituent in the atmosphere of Venus, and that it
may well be the predominant constituent.

In Chapter IV we considered the evolution of the
Earth's atmosphere in some detail. We came to
the conclusion that most of the atmosphere that the
Earth originally possessed was lost very early, whilst
the Earth was still molten. As the Earth cooled and
solidification set in, large quantities of gases,
principally water-vapour and carbon dioxide, must
have been evolved from the semi-molten mass.
These gases with the residual gases from the initial
atmosphere, consisting mainly of nitrogen, argon,
neon and probably some carbon dioxide, formed the
new atmosphere.

The same general course of evolution is likely to
have occurred in the case of Venus, which is so
nearly equal to the Earth in size and weight. The
atmosphere of Venus, in the early stages after
solidification set in, consisted of carbon dioxide and
water-vapour, along with nitrogen, argon, neon and
probably small but relatively unimportant quanti-
ties of other gases. Nitrogen, being a left-over
from the initial atmosphere, is probably present in
the atmosphere of Venus in smaller amount than in
the atmosphere of the Earth, because it would be
somewhat easier to escape from Venus than from
the Earth.

When Venus cooled to a temperature below the
boiling-point of water, most of the water-vapour
would condense out of the atmosphere and form
oceans and lakes, leaving an atmosphere consisting

VENUS THE EARTH'S TWIN SISTER 169

primarily of carbon dioxide and nitrogen the
quantity of nitrogen being less, however, than that
in the Earth's atmosphere with some argon and
small quantities of neon and other gases.

The Earth a,t one stage in its history had an
atmosphere of a similar nature, but we have seen
that the atmosphere of the Earth then passed
through a further stage, which consisted essentially
in the removal of the carbon dioxide and its replace-
ment by oxygen. It is clear that the atmosphere of
Venus has not passed through this stage, at any rate
to any great extent. The abundance of carbon
dioxide in its atmosphere and the scarcity, or
possibly even the absence, of oxygen are both
confirmatory.

This is a very important and significant con-
clusion. What does it tell us ? When considering
the atmosphere of the Earth, we remarked that the
presence of free oxygen demanded explanation
because oxygen is chemically a very active sub-
stance and there are agencies in continual action
tending to deplete the supply. The only way in
which it was possible to account for the presence of
free oxygen in the atmosphere of the Earth was
through the action of vegetation, and it appeared
that the store of oxygen in our atmosphere has been
brought about through vegetation being buried and
thereby preserved from decay vegetation that is
now represented by the coal and oil beneath the
surface of the Earth. If there were vegetation on
Venus to any great extent, the atmosphere of Venus
would also have passed through the corresponding
stage of evolution. It clearly has not done so.

I7O LIFE ON OTHER WORLDS

Venus is, in fact, very much like what the Earth was
before life had commenced to develop.

We can draw a picture of Venus that is probably
pretty near the truth, despite the fact that we have
never seen her surface. An atmosphere rich in
carbon dioxide will have a very powerful blanketing
effect. This effect, together with solar radiation
stronger than the Earth receives, will combine to
make the temperature at the surface of Venus con-
siderably higher than the temperature of the Earth.
It is likely that the temperature is not much below
1 00 C., the temperature at which water boils.
Under such circumstances it is most improbable
that life would begin to develop. The lack of
vegetation and the absence of oxygen from the
atmosphere of Venus are thereby explained. There
are doubtless extensive oceans and swamps and a
very hot humid atmosphere; the abundance of
moisture is responsible for the permanent layer of
thick clouds.

Venus 5 then, appears to be a world where life has
not yet developed, or, if it has commenced, where
it is merely in such a primitive stage that we cannot
obtain any direct evidence of it. It is a world
where conditions are not greatly different from
those that existed on the Earth many hundreds of
millions of years ago. There may be expectations
of life in the remote future when, as the Sun's
supply of radiation becomes gradually depleted and
the Sun slowly cools down, conditions will approxi-
mate more and more to those that the Earth passed
through and which led eventually to the appear-
ance of life. As conditions become more suitable

VENUS THE EARTH'S TWIN SISTER 171

for life to appear on Venus they will become less
favourable for its continued existence on the Earth.
After life on the Earth has become extinct, a new
chapter may commence on Venus leading gradually
and progressively to more and more highly devel-
oped forms of life and ultimately who can tell ?
to intelligent life.

CHAPTER VIII

MARS THE PLANET OF
SPENT LIFE

To many persons Mars is the most interesting object
in the heavens because it is the one and only world
where we appear to have direct evidence of life an5
because some astronomers have held the opinion
that it provides evidence for the existence on it of
intelligent beings.

Mars revolves in an orbit that is outside the orbit
of the Earth, its mean distance from the Sun being
a little more than one and a half times that of the
Earth. The orbit is rather elliptical and, in conse-
quence, the distance of Mars from the Sun varies by
more than 26 million miles. It requires a period
of a little short of two years for Mars to complete
one revolution in its orbit, and we overtake it on
an average once in every two years and fifty days.
Mars is then said to be in opposition, because the Sun 3
the Earth and Mars are nearly in a straight line
(they would be exactly in a straight line if it were
not for the fact that the orbits of Mars and of the
Earth are not quite in the same plane), the Sun and
Mars being on opposite sides of the Earth, so that
Mars rises at sunset and crosses the meridian at
midnight. Because the orbits of Mars and the
Earth are both somewhat elliptical the distance of
Mars from the Earth at opposition can range from
about 35 million miles to about 63 million miles.
The nearer Mars is to the Earth at opposition, the

172

MARS THE PLANET OF SPENT LIFE 173

more favourable the opportunity for studying its
surface. The most favourable oppositions occur in
August and the least favourable in February. The
planet is at its brightest at opposition. At the least
favourable oppositions Mars is not quite as bright
as Sirius, the brightest star in the sky, whilst at the
most favourable it becomes much brighter than any
star and brighter than any other planet at its
brightest except Venus. When at its greatest
distance from the Earth, Mars is about half as
bright again as the pole star.

The diameter of Mars is about 4,215 miles, only
a little more than half that of the Earth. Its weight
is rather more than one-tenth of the weight of the
Earth. The force of gravity on its surface is only
two-fifths as great as that on the surface of the Earth
and the velocity of escape from Mars is about three
miles a second. This is less than one-half of the
velocity of escape from the Earth, and we should
therefore expect to find that Mars has some atmo-
sphere, but that this atmosphere will prove to be
considerably more tenuous and less extensive than
the atmosphere of the Earth.

The opportunities for the satisfactory observation
of Mars are somewhat limited. Its apparent
diameter ranges from 3! seconds of arc, when at
its greatest distance, to twenty-five seconds at the
most favourable opposition. At the favourable
oppositions the size of the image seen in a telescope
is therefore about seven times greater, and the area
of the image about fifty times greater than when the
planet is at its greatest distance. For the study of
fine detail on the surface of the planet, the condi-

174 LIFE ON OTHER WORLDS

tions are really only satisfactory for a few months
around opposition 01% in other v,ords, for a few
months every two years or so.

Suppose we have at our command a large tele-
scope of twenty-five feet focal length. At the most
favourable oppositions, the diameter of the image of
Mars in the focal plane of the telescope is only ^
inch; at the least favourable oppositions, it is about
half as large whilst, when at its greatest distance,
the diameter of the image is only ^fa- G inch. The
smallness of the image in even a large telescope makes
it impossible to study in minute detail the surface
markings on Mars by photography. The detail is
so intricate that much of it is finer than the grain of
the photographic plate; moreover, the planet is
never bright enough to be photographed by an
instantaneous exposure. A time exposure is needed
and then the slight tremors in the atmosphere,
which almost invariably are present in greater or
less degree, blur out the finer details in the image.
If we endeavour to reduce the trouble arising from
the coarseness of the grain of a fast plate by using
slow fine-grained plates, it becomes necessary to
increase the time of exposure considerably and the
troubles from atmospheric unsteadiness then become
greater. So in either case there is a limit to what
the photograph can reveal. That is the reason
why photographs of Mars show less detail than is
recorded in drawings by competent observers. In
visual observations, it is possible to wait for the
occasional instants when the atmosphere becomes
momentarily steady and the detail sharply defined,
for on most nights there occur brief moments when

MARS THE PLANET OF SPENT LIFE 175

the seeing conditions become much better than
the average.

When Mars is at its nearest to the Earth, it is
more favourably placed for observation than any
other heavenly body, with the exception of the
Moon. It is true that Venus at times comes closer
to the Earth than Mars ever does but, at such times,
Venus appears as a very narrow crescent and is
visible only by daylight. If, when Mars is most
favourably placed, we observe it in the telescope
with a power of seventy-five, it appears as large as
the Moon does to the naked eye. It might seem at
first sight that the detailed study of the surface
under such conditions would be easy. But if we
compare the coarse detail that we can see on the
Moon with the naked eye with the intricate fine
detail shown on a photograph obtained with a large
telescope, it will be realised how much is lost.

Mars exhibits slight phases in the telescope, which
were discovered by Galileo in 1610. But, since its
orbit lies outside the orbit of the earth, it can never
appear as a crescent, like Mercury and Venus.
The greatest phase shown by Mars is comparable
with the phase of the Moon when it is three days
from full.

Under favourable conditions Mars appears in the
telescope as a beautiful object, with a strong orange
colour, on which misty markings can be seen. The
first sketch showing surface markings on Mars was
made in 1659 by Huyghens, who, by studying the
apparent movements of these markings, suggested
that Mars rotated in twenty- four hours. In 1666
Cassini found the period of rotation to be twenty-

176 LIFE ON OTHER WORLDS

four hours forty minutes; this is close to the period
given by modern observations, which is about
twenty-four hours thirty-seven and a half minutes.
Cassini also observed the characteristic feature of
the polar caps, but it was not until near the end of
the eighteenth century that Sir William Herschel
detected the variation of the size of the polar caps
with the seasons.

Around whichever of the poles of Mars happens
to be visible there is seen a bright white cap. The
two polar caps show regular seasonal changes in
size. During the northern summer, for instance,
the northern cap shrinks whilst the southern cap
grows. With the changing of the seasons the
northern cap grows again whilst the southern cap
shrinks. Analogy strongly suggests something simi-
lar to the regions of ice and snow around the north
and south poles of the Earth.

In contrast to the changes in the polar caps,
which are easily seen in moderate-sized telescopes,
the dark markings are more 0? less permanent.
We see these markings carried round by the rotation
of Mars, and this enables the period of rotation to
be fixed with high accuracy.

The first really detailed and careful survey of the
surface of Mars was made by the Italian astronomer,
Schiaparelli, at the favourable opposition of 1877.
Schiaparelli was a highly competent observer, he
had an excellent telescope, good conditions for
observing, and Mars was unusually close to the
Earth. The existence of dusky markings on the
planet, which stood out against the ruddy back-
ground, was already known. It was believed that

MARS THE PLANET OF SPENT LIFE 177

these dusky markings were seas and that the ruddy
background was dry land. But in 1877 Schiaparelli
discovered, what had not previously been known,
that there were dusky streaks crossing the land
areas or " continents/' connecting up the " seas "
with one another. He termed these streaks canali
which, interpreted literally, means channels. But
the similarity of the Italian word to the English
word canal has caused a narrower interpretation to
be placed upon the term given by Schiaparelli than
he intended, with the result that there has been a
good deal of misrepresentation.

Schiaparelli continued to observe Mars for a
number of years and discovered that the seas were
not uniformly dusky. He remarked that the colour
of the seas was generally brown, mixed with grey,
but not always of equal intensity in all places, nor
always the same in the same place at all times.
From an absolute black it might descend to a light
grey or to an ash colour. He compared these differ-
ences in colouration on Mars with differences in
colour of the seas on the Earth, adding that the
seas of the warm zone are usually much darker than
those near the pole; the Baltic, for instance, has a
light muddy colour that is not observed in the
Mediterranean. Schiaparelli noticed that some at
least of the changes in colouration were seasonal in
nature; he interpreted these changes as being
changes in colour of the seas, which became darker
as the Sun approached their zenith and summer
began to rule.

It was noticed also by Schiaparelli that the so-
called continents were not uniform in colour. Over

12

178 LIFE ON OTHER WORLDS

the bulk of the continents an orange colour pre-
dominated which in some areas, of relatively small
extent, reached a dark red tint, whilst other small
regions were yellow or white. But besides the dark
and light regions, which were considered to be seas
and continents respectively, there were some areas
of small extent that were of an amphibious nature,
appearing sometimes yellowish like the continents,
sometimes brown or even black like the seas,
whilst in other cases the colour was intermediate in
tint, leaving a doubt whether they were sea or land
areas. He concluded that these represented huge
swamps, in which the variation in the depth of the
water produced the diversity of colour.

The vast extent of the continents was furrowed
upon every side by a network of numerous lines or
fine stripes of a more or less pronounced dark
colour, whose aspect was very variable. They
traversed the planet for long distances in regular
lines, not at all resembling the winding courses of
rivers on the Earth. Some of the shorter ones
were only a few hundred miles in length, but others
extended for thousands of miles, extending over as
much as one- third of a circumference of Mars.
Some of these lines or channels (canali) were very
easy to see; others were extremely difficult and
resembled the finest thread of spider's web drawn
across the disk. The breadth of some may be as
great as one or two hundred miles; of others, not
more than twenty miles.

The conclusion that Schiaparelli drew from his
long-continued observations was that these channels
were fixed configurations on the planet. Their

MARS THE PLANET OF SPENT LIFE 179

length and arrangement were constant or varied
only within narrow limits, and each one began and
ended between the same regions. But their ap-
pearance and degree of visibility varied greatly
from one opposition to another and even from one
week to another. The variations in the appearance
of the different channels were not simultaneous,
but appeared to occur capriciously, so that one
might become indistinct or even invisible whilst
another in its vicinity became at the same time
conspicuous. The channels intersected one another
at all possible angles, but usually at the small dark
spots, which Schiaparelli interpreted as lakes. Every
channel opened at its end into either a sea, or a
lake, or into another channel. None of them was
cut off in the middle of a continent, remaining
without beginning or end.

His considered conclusion in 1893 was that the
canali were truly great furrows or depressions in the
surface of the planet, destined for the passage of
water. The changing appearance of the channels he
attributed to inundations resulting from the melting
of the snows, followed by the soaking away of the
water and its eventual drying up. He added that
the network of the channels was probably a
geological formation and that it was not necessary
to suppose them to be the work of intelligent beings.

The most surprising feature about the canals (we
shall henceforth use the term that has come into
general use) was their doubling. This occurred,
according to Schiaparelli, who first announced it
in 1882, principally in the months preceding and
following the melting of the polar cap. In the

l8o LIFE ON OTHER WORLDS

course of a few days, or even of a few hours, a canal
would change its appearance and be transformed
throughout its length into two lines or uniform
stripes, more or less parallel to one another, which
ran straight and equal with the exact geometrical
precision of the two lines of a railroad. The two
canals, he asserted, followed very nearly the direc-
tion of the original canal and ended where it ended.
One of the two might follow the course of the
original canal or it might be that they would lie on
either side of it. The distance between the two
canals might range from about thirty miles to three
or four hundred miles.

The doubling did not occur at the same time for
all those canals that showed the phenomenon, but
was produced here and there, in an isolated irregular
manner and without any recognisable order. At
different oppositions, the doubling of the same canal
produced different appearances, as to width,
intensity and arrangement. It was therefore con-
cluded that the doubling could not be due to a
fixed formation on the surface of Mars, of a geo-
graphical character like the canals.

Schiaparelli added: " Their singular aspect, and
their being drawn with absolute geometrical pre-
cision, as if they were the work of rule or compass,
has led some to see in them the work of intelligent
beings, inhabitants of the planet. I am very careful
not to combat this supposition, which includes
nothing impossible. 53 He went on to consider
various suggested explanations that had been put
forward to account for the appearance and con-
cluded: "The examination of these ingenious

MARS THE PLANET OF SPENT LIFE l8l

suppositions leads us to conclude that none of them
seem to correspond entirely with the observed facts,
either in whole or in part. Some of these hypotheses
would not have been proposed, had their authors
been able to examine the doubling with their own
eyes. Since some of these may ask me directly,
Can you suggest anything better ? I must reply
candidly, No."

The conclusions reached by Schiaparelli from his
long-extended observations of Mars have been given
in some detail because they had the effect of giving
an immense stimulus to the study of Mars. The
news that changes could be seen occurring on the
surface of Mars was interpreted by some as provid-
ing evidence that there were intelligent beings on
Mars for, it was argued, the numerous canals
following straight or regular paths could not be
natural formations, but must have been artificially
made. This interpretation was violently combated
by others; Schiaparelli, as we have seen, kept an
open mind on this question; he did not accept it
as proved, though he did not regard it as impossible.

The great protagonist of the theory of the artificial
nature of the canals was the American, Percival
Lowell, who in 1894 founded an observatory at
Flagstaff, in Arizona, for the special purpose of
studying the planets, and Mars in particular. The
site for the observatory, at a high altitude in the
dry region of Arizona, was selected for the excellence
of the atmospheric conditions. There, for many
years, Lowell and his assistants studied Mars
assiduously, whenever it was sufficiently well placed
for observation, and accumulated a mass of in-

l82 LIFE ON OTHER WORLDS

formation about the changes that occur on its
surface.

One of the early discoveries made at the Lowell
Observatory was that the dark areas of the planet,
which had hitherto been regarded as seas, showed
a considerable amount of detail and that they were
crossed, like the ruddy areas, which had hitherto
been regarded as continents, by canals. Changes
both of colour and form occurred in these dark
areas and it was concluded that such changes were
primarily of a seasonal character. These results
provided conclusive evidence that the dark areas
could not be seas, in the ordinary sense of the word.
If they were watery areas, they must be more in the
nature of marshland. The conclusion to which
Lowell came was that the dark areas were regions
where there was vegetation, the fertile regions of
Mars, in contrast to the ruddy-coloured regions,
which represented the arid desert regions, where no
vegetation could grow.

The changes that were found to occur in the dark
areas were of two kinds : some were quite irregular
whilst others were considered to be seasonal in
character. There can be no doubt that at times
well-marked changes occur over considerable areas,
which are not of a seasonal nature. Thus, for
instance, a certain marking observed by Schiaparelli,
and named by him Lacus Mceris^ could not be found
by Pickering at Arequipa in 1892. It reappeared
with perfect distinctness, and was observed by
Lowell, in 1903, after an interval of thirteen years
since it had last been seen. Another marking,
called Lacus Solis, showed a marked change between

MARS THE PLANET OF SPENT LIFE 183

1924 and 1926, being extended much farther to the
north in the later year; by 1928, it had recovered
its normal appearance. Such large-scale changes
are relatively infrequent.

The visibility of many other markings was found
by Lowell to change in a regular seasonal manner.
The general character of these changes were the
same. Soon after the beginning of the melting of
the ice-cap in the summer hemisphere, the canals
begin to become visible in the polar regions adjacent
to that cap. The darkening of the canals spreads
gradually and progressively towards the equator
and beyond it, into the opposite hemisphere, at the
rate of about fifty miles a day. At the same time
occur changes of colour of the dusky markings,
from a light to a darker green and later to brown
and yellow. These changes were attributed by
Lowell to a transference of water from the melting
pole cap towards and beyond the equator, the
transference being accompanied by the growth of
vegetation.

These simple and regular changes, proceeding in
harmony with the progression of the seasons on
Mars, have not been altogether confirmed by other
observers. It might be thought that regular
changes, recurring with the seasons, would be easy
to establish. It must be remembered, however,
that at any single opposition Mars is sufficiently well
placed for observation for only a few months; as
the length of the Martian year is 687 days, the
advance in the seasons during the time over which
observations can extend at any one opposition is
not very great. In order to observe Mars through-

184 LIFE ON OTHER WORLDS

out one complete cycle of the seasons, it would be
necessary to continue observations for fifteen years.
Moreover, because the rotation of Mars is about
37 minutes slower than the rotation of the Earth,
there is a slow falling behind in the longitudes of
Mars presented centrally to the Earth at the same
hour on successive nights. In consequence, any
given marking is only well placed for observation
for about a fortnight consecutively, during which
time there may be only one or two nights with good
conditions for observation. After this time the
marking becomes unfavourably placed at the hours
suitable for observation and cannot be observed
again for about a month.

It will be clear that it is by no means easy to
follow the changes in the surface markings through
a complete cycle of the Martian seasons. A further
complicating factor is that it is not possible to take
advantage of photography to give a permanent
record of the appearance of the planet at a given
instant because, as we have already explained,
most of the detail on the surface is too fine to be
recorded by the photographic plate; the record of
the appearance of the planet must, therefore, be
provided by the observer drawing what he sees. A
good observer is not necessarily a good draughts-
man, however, and conversely the good draughts-
man may not be a good observer. But however
good the draughtsman, it is by no means an easy
matter to portray faithfully the detail seen on the
face of the planet, in its varied light and shade, with
proper contrasts of colour and with the positions
and sizes of the various markings shown in their

MARS THE PLANET OF SPENT LIFE 185

proper relationship one to another. Much of the
detail that the observer is striving to portray is at the
limit of vision and only to be glimpsed momentarily
at rare intervals. In the circumstances, it would
not be surprising to find that the observer had
tended at times to concentrate his attention on
certain details and at other times to concentrate it
on different details. The difficulty of excluding the
possibility that seasonal changes in fine detail do not
arise in this way, and that they are not of subjective
origin, will be appreciated.

Lowell claimed also to have observed the doub-
ling or pairing of certain canals, previously
announced by Schiaparelli, as we have already
mentioned. He asserted that the majority of the
canals were persistently and perpetually single, but
that a proportion of them at times appeared mysteri-
ously paired, the second canal being an exact
replica of the first, running by its side throughout
its whole length and keeping equidistant from it,
like the two lines of a railway track. The distance
between the two canals of a pair varied, according to
Lowell, from about seventy-five miles to 400 miles.

We shall now consider briefly the interpretation
placed by Lowell upon his observations. He
suggested that we must try to see ourselves as others
see us. Suppose that we could do away with the
clouds that at any moment cover much of the sur-
face of the Earth and that we looked at the Earth
from Mars or Venus. From such a distance the
local merges into the general aspect and, at intervals
of six months, an interesting and beautiful trans-
formation would be seen to spread over the face of

l86 LIFE ON OTHER WORLDS

the Earth the vernal flush of the Earth's awakening
from its winter sleep. This would be revealed
through the growth of vegetation starting in temp-
erate latitudes and moving progressively toward the
pole, the tint deepening the while. The wave of
awakening on the Earth would travel from equator
to pole, whereas on Mars it journeys from pole to
equator. The difference, according to Lowell, was
of fundamental significance for the interpretation of
the changes observed on Mars. The growth of
vegetation is dependent, provided certain other
essential requirements are satisfied, on the Sun's
rays and the presence of moisture. On the Earth
water is almost always available, except in desert
regions, but, unless it is called by the Sun, vegetation
never wakes. After the Sun has departed south in
the autumn, the vegetation in northern latitudes
must await its return in the spring.

Mars, according to Lowell, was otherwise circum-
stanced. Not merely was the warmth from the Sun
needed to awaken the vegetation to growth but it
was also needed indirectly to provide the water
supply. There is no surface water on Mars and the
annual unlocking of a water supply, provided by
the melting of the snows of the polar cap, was needed
before the vegetation could commence to grow.
The growth must, therefore, start at the pole,
where the water supply first becomes available, and
it then follows the frugal flood towards the equa-
torial regions.

The next step in the argument was that though
vegetable life could reveal itself directly, animal life
could not. " Not by its body, but by its mind

MARS THE PLANET OF SPENT LIFE 187

would it be known. Across the gulf of space it
could be recognised only by the imprint it had made
on the face of Mars." This imprint he recognised
in the canals, the long straight markings, which
were just such markings as intelligence might have
made. The unnatural regularity of these markings,
as Lowell drew them, made it impossible to believe
that they could be natural features. There was the
additional fact that many of the canals converged
to dusky patches at their junctions, the so-called
oases, and passed from one oasis to another in an
absolutely unswerving direction. " The observer
apparently stands confronted with the workings of
an intelligence akin to and therefore appealing to
his own. What he is gazing on typifies not the out-
come of natural forces of an elemental kind but the
artificial product of a mind directing it to a pur-
posed and definite end. 35

He therefore concluded that the canals were
artificial channels, made by intelligent beings, to
carry the melting water from the poles across the
surface of the planet, passing from point to point by
the shortest possible paths. It follows, if this inter-
pretation is correct, that Mars must be a world
devoid of mountains. As the water travels along
these channels, the irrigation makes it possible for
vegetation to spring up along their banks and where
the canals meet, at the oases, are the fertile regions
where the Martian beings live.

Some explanation is required of how the water
manages to flow from the pole to the equator and
beyond. It cannot be flowing down hill all the
way; if it were down hill from the pole to the

l88 LIFE ON OTHER WORLDS

equator, the water would be running uphill after
crossing the equator. Therefore, the water must
have been artificially conducted over the face of the
planet. There must be a great pumping system, on
a scale far surpassing any of the works of man, and
this in itself presupposes an advanced type of in-
telligence. Lowell calculated that the power re-
quired would be four thousand times the power
of the Niagara Falls.

And what is the motive for this gigantic irrigation
system ? That is readily found. It is provided by
the instinct for self-preservation on the part of a
world that is becoming increasingly arid. In the
struggle for existence, water must be got. The
Martians saw in the growing scarcity of water the
premonition of their doom. All other questions
became to them of secondary importance in com-
parison with the vital urge to obtain water. The
only places where water is in storage and whence it
may be got are the poles : hence the whole economy
of life on the planet must centre round making this
water available for the needs of life. As the great
occupation of the Martians must accordingly be
that of getting water, is it to be wondered at that it
is the fruits of this occupation that have revealed
their existence to the eyes of man ?

Only with an intelligent population, but not
otherwise, would the inevitable progressive desicca-
tion of the planet be foreseen. The water supply
would not fail in a moment; it would be a slow
gradual process. Local needs would urge the
reaching out to a distant supply, as is already being
done on our Earth for the provision of adequate

MARS THE PLANET OF SPENT LIFE 189

water supplies for large towns and cities. The steps
to greater and still greater distances would follow
one by one until eventually the surface of the planet
was covered with a vast network of channels,
making water available and enabling vegetation to
grow.

Lowell concluded his book, entitled Mars as the
Abode of Life , with these words: " A sadder interest
attaches to such existence: that it is, cosmically
speaking, soon to pass away. To our eventual
descendants life on Mars will no longer be something
to scan and interpret. It will have lapsed beyond
the hope of study or recall. Thus to us it takes on
an added glamour from the fact that it has not long
to last. For the process that Brought it to its present
pass must go on to the bitter end, until the last spark
of Martian life goes out. The drying-up of the
planet is certain to proceed until its surface can
support no life at all. Slowly but surely time will
snuff it out. When the last ember is thus extin-
guished, the planet will roll a dead world through
space, its evolutionary career forever ended. 55

Such, in brief, was Lowell's theory: attractive,
ingenious and logical, provided that the observa-
tional basis can be accepted. But that is where the
rub came, for though there were observers of Mars,
many of them possessed of moderate-sized instru-
ments, who confirmed Lowell's observations, there
were other observers who were unable to see the
essential phenomena upon which his theory was
based ; some of these were observers of great acuity
of vision and of considerable reputation, with large
instruments placed where the conditions for observa-

LIFE ON OTHER WORLDS

tion were particularly favourable. The whole
question of the nature of the detail to be seen on
Mars and of its interpretation became a matter of
violent controversy. Time has passed; the con-
troversy has died down; there is now a general
consensus of opinion about what can be seen on
Mars. Let us take stock of the situation and see
what we can fairly accept as established beyond the
possibility of doubt.

The main argument in favour of the Martian
civilisation was the artificial appearance of the net-
work of canals shown on the maps of Mars, com-
piled by Lowell and others. There is no denying
that the maps do have a very artificial appearance.
But they are compiled from many separate drawings
and all the canals shown on a map are never seen at
once; only a few are visible on any one night.
Nevertheless a few canals are sufficient to give a
very artificial appearance. It must be emphasised,
on the other hand, that though the drawings may
have been most carefully made, the planet itself if
we could approach sufficiently near it would
doubtless look entirely different in its main features
from any of the drawings and would not appear at
all artificial.

The question at issue is not whether the canals
exist or not. There can be no question about the
existence of at least the most conspicuous of them.
Some can be seen in telescopes of moderate size and
a few have been photographed. The photographs
reproduced in Plate 14, perhaps the best photographs
of Mars that have been obtained, show some of
the canals clearly. The question about which

MARS THE PLANET OF SPENT LIFE

there has been so much controversy is whether
these features of the Martian surface are the
straight, narrow, sharply defined lines, which they
appear to be in Lowell's drawings. Dr. Barnard,
an experienced observer with a very keen eye,
who had the advantage of observing with some of
the largest telescopes in America, saw them as
ill-defined, irregular, diffuse shadings, which were
not of uniform breadth and were not always even
continuous. Observing with the great sixty-inch
reflecting telescope at Mount Wilson, a telescope
much more powerful than Lowell had at his dis-
posal, he said that Mars gave " the impression of a
globe whose entire surface had been tinted a slight
pink colour, on which the dark details had been
painted with a greyish coloured paint, supplied with
a very poor brush, producing a shredded or streaky
and crispy effect in the darker regions. " He
added that " no one could accurately delineate the
remarkable complexity of detail of the features
which were visible in moments of the greatest
steadiness. 35 M. Antoniadi, who has studied the
planets for many years with the great refractor at
Meudon, agrees with Barnard. The general con-
sensus of observational opinion, in fact, is in agree-
ment with Barnard that the canals do not form a
sharply defined geometrical network but that they
are broad, diffuse and irregular in outline.

The appearance of the canals has been described
by Dr. Waterfield, who is Director of the Mars
Section of the British Astronomical Association, and
who has observed Mars for many years. He states
that " when he first started observing Mars he

IQ2 LIFE ON OTHER WORLDS

found that he was inclined to see more or less
linear detail upon the disk. But as time passed
that tendency grew less and less, and he began to
see the detail more like what he now considers to
be its actual appearance. It undoubtedly takes
many years for the eye to become trained for the
appreciation of the finest telescopic detail. Before
that is accomplished, it is liable to see hazy dis-
continuous streaks as narrow and continuous lines,
to interpret a complex system of light and dark
shadings as a more or less linear geometrical pattern,
and even to join up the longer and quite obvious
markings by lines that do not possess any objective
reality. He finds that this tendency is still liable
to recur when the planet is far from the Earth and
difficult to observe, when our atmospheric conditions
are poor, and when telescopes of smaller size are
used. Under normally good conditions he sees
the canals as wide and diffuse streaks which often
form the border of a more extensive shading and
are generally irregular in width and sometimes
discontinuous. Finally, under the very best at-
mospheric conditions which occur only momen-
tarily on comparatively few nights in the year
some of these streaks remain while others melt
away into a background of more complex structure."
There is a subjective tendency for the eye to
connect up detail in the form of irregular shadings
and markings, which are almost at the limit of vision
and glimpsed only with difficulty, by continuous
lines. The following experiment is easily made.
Draw a row of dots, one-eighth of an inch apart,
on a piece of paper and look at it from a distance

MARS THE PLANET OF SPENT LIFE 193

of about thirty feet. The separate dots will not be
seen; they will appear to form a continuous,
uniform line. Because the canals appear to be
continuous or because some of them have been
recorded on photographs it does not necessarily
follow that they are continuous.

It must not be thought that there is any question
of dishonesty or bad faith on the part of the ob-
servers to account for, such extremes as the deli-
neations of Lowell and Barnard. These two
observers studied Mars for many years under
favourable conditions and both were trained and
experienced observers; each of them has honestly
recorded the appearance of the planet as he saw it.
The only possible explanation of the differences is
that the observation of these faint elusive details is
subject to complex personal differences. The ob-
server looks in the telescope and an image is formed
on the retina of the eye. He has to interpret what
he sees and to transfer the impression in his mind
into a drawing. Differences of visual acuity must
certainly be an important factor. One observer
may look at the image of a twin star, whose two
components are separated by a very small amount,
and see clearly that the star is double; another
observer will see the star as a single star. Sub-
conscious interpretation of what is faintly glimpsed
may be very different for two different persons.
The eye of one may tend to bridge the gap between
faint details and to draw a marking as a uniform,
straight, continuous line unless he can clearly see
that there are irregularities, bends and discontinu-
ities in it. Another may only draw it in this way

13

194 LIFE ON OTHER WORLDS

when he can see beyond the possibility of doubt
that it is uniform, straight and continuous.

The truth may lie somewhere between the two
extremes, though it appears probable that Barnard's
delineation is the nearer approach to the truth. If
the canals are as sharp and straight as Lowell
depicted them, it seems incredible that other
observers, if they could see them at all, should not
also see them as sharp and straight. The experi-
ment was tried of placing a diagram, on which
there were scattered a number of dots of various
sizes, short lines and shady patches in an irregular
manner, before a class of school children, who were
told to draw what they could see. Many of them,
particularly those at the back of the room, con-
nected up the more prominent features with straight
lines. It is much more likely that the eye will tend
to connect up disconnected structure in this way
and to represent it as continuous than that it will
tend to break up continuous structure into dis-
connected patches.

Lowell's representation of paired canals seems to
provide positive proof that his observations were
liable to subjective error. If two lines are drawn
in ink on white paper and viewed from a distance
with a 6-inch telescope, it is not possible for the
eye to separate them unless the angular separation
is at least one second of arc. This arises from the
fact that, for optical reasons, the telescopic image
of a sharp line is not perfectly sharp; the image is
somewhat broadened : if there are two lines suffi-
ciently close together, the images will converge into
one another so that no eye, however acute, can

MARS THE PLANET OF SPENT LIFE 195

possibly separate them. The limiting separation of
one second of arc, for a 6-inch telescope, is inferred
from optical theory and confirmed by observation.
Lowell, however, with a six-inch telescope recorded
paired canals whose separation was as small as
0-26 second. His own assistant expressed grave
doubts about the objective reality of the duplication,
and other experienced observers have not been able
to confirm the existence of these paired canals.

The observations and deductions of Lowell have
been described in some detail because of their great
interest for the present purpose of considering the
evidence for or against life on the planets. The
conclusion which it seems reasonable to accept is
that the geometrical network of narrow straight
canals does not exist. There is no doubt that there
are faint markings in the form of hazy streaks,
which are fairly straight and seem to be pretty
continuous. Under conditions of exceptional steadi-
ness, they appear to resolve into finer and more
complicated detail. It is doubtful, therefore,
whether these markings are really continuous and
it is idle to speculate about their nature. We must
abandon Lowell's theory of an artificially con-
structed network of water channels and accept the
canal markings as natural formations. The dif-
ference in the topography of the Earth and the
Moon serves as a reminder that other worlds need
not necessarily bear any close resemblance to the
Earth in surface features. It was suggested by
Dr. W. H. Pickering that the canal formations
might be due to volcanic cracks lying between
craterlets on the Martian surface; water-vapour

196 LIFE ON OTHER WORLDS

might escape from these craterlets and cracks and
nourish vegetation growing along their sides; it is
this vegetation and not the crack itself that would
be visible in the telescope.

We must lay aside all these speculations and
preconceived notions, however captivating to the
imagination they may be, and consider without bias
what information about Mars has been substan-
tiated by further observation and what conclusions
can legitimately be drawn from it.

We consider first what direct observational
evidence there is of an atmosphere on Mars. In
our survey of the solar system we have so far
encountered some worlds, such as the Moon and
Mercury, whose surface we can see but which are
devoid of an atmosphere, and other worlds, such
as Venus and the major planets, where there is
ample evidence of an atmosphere but where we are
unable to see the solid surface. When we come to
Mars, we are able to see the surface and we expect,
as we have already mentioned, to find an atmo-
sphere. The seasonal change in the size of the
polar caps provides indirect evidence that there
must be an atmosphere. When the polar cap melts
with the advance of the summer season, there must
also be some evaporation of moisture; if Mars had
entirely lost its atmosphere, it must also have lost
this water-vapour in the course of ages and the
material of which the polar caps are formed would
gradually have been completely dissipated away
into space.

But we do not have to rely, fortunately, merely
on indirect evidence of an atmosphere on Mars.

MARS THE PLANET OF SPENT LIFE IQ7

Direct evidence of an atmosphere was provided by
Dr. Wright who, at the Lick Observatory, by using
appropriate colour filters, photographed Mars by
the light of different colours. Photographs by
infra-red light, which has penetrating or haze-
cutting properties, showed the surface detail clearly;
the photographs in the ultra-violet light showed
practically no trace at all of surface features.
This is illustrated in Plate 12, which shows a ter-
restrial landscape and Mars photographed in infra-
red and ultra-violet light respectively. The ter-
restrial landscape represents the view from the top
of Mount Hamilton, looking towards the distant
mountains across the intervening valley. In the
infra-red photograph the detail is clearly shown,
but in the ultra-violet photograph all that can be
seen is a faint outline of the crest of the distant
mountains. There was sufficient haze in the
atmosphere to scatter the light of short wave-length
to such an extent that all detail was lost. The
comparison of the two photographs of Mars, which
were taken on plates identical with those used for
the terrestrial photographs, shows that the ultra-
violet light is scattered by a Martian atmosphere
to such an extent that it is not able to penetrate to
the surface of the planet and out again, whereas the
infra-red light, which, because of its longer wave-
length, is not scattered so much, can get through the
atmosphere to the surface of the planet and out
again.

By employing special filters, Dr. Wright also
photographed Mars with red, yellow, green and
blue light and found that the surface detail became

198 LIFE ON OTHER WORLDS

less and less distinct as the wave-length of the light
used became shorter and shorter. The whole series
of photographs provides a progressive sequence from
the infra-red photograph in which the surface detail
is clearly seen, the detail becoming progressively less
distinct until in the ultra-violet photograph there is
practically no evidence of any surface features.

The photographs revealed another interesting
fact. The images of Mars on the photographs taken
with the ultra-violet light are larger than those
taken with the infra-red light. This is clearly
shown in Plate 14. In the latter case we obtain
an image of the solid planet itself; in the former
case we have an image of the atmospheric shell
surrounding Mars. The difference in size of the
two images corresponds to a difference of fifty
or sixty miles in the true radii, so that the atmosphere
of Mars has a considerable depth. Comparison
with terrestrial photographs taken under favourable
conditions suggests that the Martian atmosphere,
though evidently of considerable depth, is never-
theless very tenuous and that the total atmospheric
pressure on Mars does not amount to more than a
few per cent, of that at the surface of the Earth.
The force of gravitation at the surface of Mars is
only two-fifths of the force at the surface of the
Earth, so that if Mars and the Earth had atmo-
spheres which, under standard conditions of pres-
sure, were of the same thickness, the atmosphere of
Mars would extend to a much greater height than
the atmosphere of the Earth.

Further confirmation of an atmosphere on Mars
is provided by the occurrence of clouds, which can

MARS THE PLANET OF SPENT LIFE IQ9

not only be seen in the telescope but can also be
photographed. The clouds are of two types, those
that appear white to the eye and those that appear
yellowish. The white clouds are best seen on the
ultra-violet photographs and are barely visible, if
seen at all, in the infra-red photographs. Such
clouds must occur fairly high up in the atmosphere
because, if they were at a low level, the ultra-violet
light would not penetrate to them; they must be
sufficiently thin to allow the infra-red light to pass
through them, otherwise they would appear more
conspicuously on the infra-red photographs. These
white clouds are somewhat rare; they have a
tendency to begin to form at about Martian noon
and to increase in size during the afternoon, as the
temperature falls. The formation and growth of
one of these clouds is illustrated in Plate 13. It
is probable that they are produced by the con-
densation of moisture, as a result of the fall in
temperature. They tend to be most conspicuous,
therefore, at sunset, when they are near the edge
of the disk. They may occasionally be seen, by
direct observation of Mars in the telescope, project-
ing beyond the edge of the disk. It then becomes
possible to make an estimate of the height of the
cloud above the surface of the planet; heights up
to twelve miles have been found in this way.

The second type of cloud, the yellow cloud, is
more frequently seen. Such clouds are shown on
the infra-red but not on the ultra-violet photo-
graphs. They must, therefore, be at a fairly low
level in the atmosphere. An example of a yellow
cloud is shown in Plate 13. These clouds appear

200 LIFE ON OTHER WORLDS

yellowish to the eye, but the lack of contrast between
them and the ruddy surface of the planet makes
them rather difficult to see. A very large area of
the planet is sometimes covered by the yellow
clouds, which obliterate partially or entirely the
underlying surface details; they may persist in the
same region for some time, occasionally for as long
as several weeks. It has been suggested that the
yellow clouds are clouds of dust raised by winds
blowing over the extensive desert areas of the
planets.

The polar caps show a surprising difference in
appearance in the photographs taken with light of
long- and short wave-length. The usual explana-
tion of the caps is that they are surface deposits of
snow or hoar frost in the polar regions, analogous
to the snow- and ice-caps of the polar regions of the
Earth. The only other white solid substance of
which they could consist is solid carbon dioxide
(commercially known as "dry ice "). Solid
carbon dioxide volatilises at low pressures, such as
must exist on Mars, at temperatures appreciably
lower than the observed temperature of the caps
and the possibility that the caps may consist of solid
carbon dioxide can, therefore, be excluded. If the
polar caps are merely surface deposits, we should
expect them to be clearly shown on the photographs
in light of long wave-length but to be invisible on
the photographs in light of short wave-length,
whereas, contrary to expectation, they are seen
most clearly on the latter photographs. The polar
caps must, therefore, be largely, though not entirely,
an atmospheric phenomenon. It is probable that

MARS THE PLANET OF SPENT LIFE 2OI

over the polar regions there are clouds, like high
cirrus clouds on the Earth, of no very great thick-
ness, so that light of long wave-length can get
through them and that there is in addition a surface
deposit of snow or ice.

This deposit cannot be of any great thickness
because, unlike the ice-caps of the Earth, it dis-
appears almost entirely in the course of the summer
months. It is quite easy to prove by calculation
that the caps cannot be very thick. The intensity
of the Sun's radiation at the distance of Mars is
known and we can calculate how thick the cap
must be in order that the whole amount of heat
received by the cap during the time that the Sun
is above the horizon, and assuming that none is lost
by reflection or radiation, will just suffice to melt it.
The thickness found on these assumptions is the
maximum possible and is only about six feet. But
most of the heat falling on the cap will be reflected
or radiated back and will not be available for melting
the ice; the true thickness must consequently be
very muchJLess than six feet and probably does not
amount to more than a few inches on the average,
except in the very near neighbourhood of the pole.
The whole quantity of water that would be ob-
tained by the melting of one of the polar caps would
not be more than sufficient to fill a large lake about
the size of Wales. A lake of such a size appears to
be entirely inadequate to supply the large quantity
of water that, according to Lowell's theory, was
pumped for thousands of miles across the surface of
Mars for purposes of irrigation.

And what about the composition of the Martian

2O2 LIFE ON OTHER WORLDS

atmosphere ? Water-vapour there must undoubt-
edly be. Although there are no qpen seas on Mars,
the existence of the polar caps and their melting as
the summer advances, together with the evidence
of clouds, afford sufficient proof that the atmosphere
must contain water-vapour. The amount of the
water-vapour is so small, however, that it can be
detected only with the greatest difficulty. The
attempts to detect it have almost invariably ended
in failure. At the Lowell Observatory, which is in
Arizona at a height of 7,250 feet above sea-level,
Dr. Slipher, in 1908, by comparing the spectra of
Mars and the Moon, when at the same altitude
and under conditions of exceptional atmospheric
dryness in the winter, found that the water-vapour
absorptions were slightly stronger in the spectrum
of Mars than in that of the Moon. This slight
difference in intensity must have been produced by
the water-vapour in the atmosphere of Mars.
Usually the absorption by water-vapour in the
Earth's atmosphere is so strong that the much more
feeble absorption in the atmosphere of Mars is
entirely masked.

All attempts to detect oxygen in the atmosphere
of Mars have been unsuccessful, and it can be con-
cluded that the amount of oxygen is not more than
one-thousandth part of the amount in the Earth's
atmosphere. Indirect evidence of oxygen is pro-
vided by the ruddy colour of Mars, which is unique
amongst the heavenly bodies. This red colour is
suggestive of rocks that have been completely
oxidised and it may be contrasted with the grey or
brownish colour of the rocks on the Moon, which

MARS THE PLANET OF SPENT LIFE 203

have remained unoxidised because of the absence of
oxygen. It appears probable that Mars may be a
planet where the weathering of the rocks, followed
by their oxidation, has resulted in the almost com-
plete depletion of oxygen from the atmosphere.

Carbon dioxide has not been detected in the
Martian atmosphere. This is not surprising be-
cause carbon dioxide must be present in large
quantity before it will produce absorptions of
sufficient strength to be detected.

The temperature of Mars is much in accordance
with what we should expect to find for a planet
somewhat more remote than the Earth from the
Sun. In the Martian tropics the temperature rises
well above the freezing-point at noon and may
reach 50 F. or a little more. The dark areas are
somewhat warmer than the reddish areas. The
observed temperature of the polar caps in winter is
very low, about 70 C.> corresponding to about
125 degrees of frost on the Fahrenheit scale. This
temperature possibly refers to the upper surface of
the cloud layer above the pole, in which case the
temperature at the surface may be appreciably
higher. At midsummer, the temperature at the
poles rises somewhat above freezing-point.

In the afternoon, as the Sun gets lower, the
temperature falls very rapidly. This is because
there is only a scanty atmosphere, with very little
water-vapour, to act as a blanket, and prevent the
escape of the long-wave heat radiation from the
surface rocks, which have been heated by the Sun
during the day. Water-vapour is very effective in
preventing the escape of the heat radiation from

2O4 LIFE ON OTHER WORLDS

the surface of a planet. Anyone who has lived in a
moist tropical climate knows that t||re is very little
fall of temperature at night, whereks in dry desert
regions, though the day temperature may be much
higher than in moist tropical regions, the night
temperature is much lower because of the rapid fall
of temperature after sunset. The maximum temp-
erature on the Earth does not usually occur at noon,
when the Sun is highest in the sky, but during the
afternoon. This is because of the blanketing action
of the water-vapour in the atmosphere, which
prevents the rapid escape of radiation from the
heated surface of the Earth and causes the temp-
erature to continue to rise for a few hours after the
Sun has reached its greatest altitude. But on Mars
the temperature is highest at noon and begins to fall
immediately afterwards. By sunset the cold has
become intense ; the minimum temperature at night
is about 130 F. The diurnal range of tempera-
ture between noon maximum and night minimum
is thus very great, being about equal to the differ-
ence between the freezing- and the boiling-points of
water.

The climate of Mars may be said to resemble that
of a clear day on a very high mountain. The solar
radiation by day is rarely tempered by cloud or
mist. By night there is rapid radiation from the
soil into space and intense cold. The climate is one
of extremes. The changes of temperature from day
to night and from one season to another are very
great. The seasons are longer than on the Earth
and their length accentuates the difference between
summer and winter conditions. The seasonal

MARS THE PLANET OF SPENT LIFE 205

changes are more pronounced in the southern
hemisphere thari in the northern. The distance of
Mars from the Sun varies by as much as twenty-six
million miles in the course of its orbital passage
round the Sun. Mars is nearest to the Sun when
it is winter in the northern hemisphere and summer
in the southern hemisphere ; it is at its farthest when
it is summer in the northern and winter in the
southern hemisphere. The southern hemisphere,
therefore, has a warmer summer but a colder winter
than the northern.

We have been compelled to discard the evidence
on which Lowell based his theory that a race of
intelligent beings exists on Mars. May there not
be sufficient evidence, however, to enable us to
conclude that there is life of some sort, but not
necessarily intelligent life, on Mars ? The temp-
erature conditions are neither so high nor so low
that the possibility of life can be excluded, though
the great daily range of temperature and the rapidity
of the changes would prove very trying for any
form of life with which we are familiar. Water-
vapour is certainly present in the atmosphere and
there is evidence of oxygen, though the supply may
be approaching exhaustion. There seems to be no
reason why life on Mars could not have adapted
itself to such conditions.

That there are changes from time to time in the
Martian surface we have already mentioned.
Some of these changes appear to be seasonal; others
are quite irregular. Lowell claimed to have
established a wave of darkening spreading equator-
wards as the ice-cap of the summer hemisphere

2O6 LIFE ON OTHER WORLDS

melted. These claims have not been altogether
confirmed by other investigators, who find the
changes neither so simple nor so cigar cut. There is
general agreement, however, that there are com-
plete changes both in appearance and colour of
various markings, which correlate with the seasonal
changes. It is difficult to interpret these changes
in any other way than by the seasonal growth of
vegetation. The vegetation covers the dark regions
of the planet, the rest of the surface being desert.
As the ice-cap melts, the moisture reaches lower
latitudes, possibly in the form of rivers or streams,
but more probably as rain or dew. With the com-
ing of the moisture, the vegetation commences to
grow and the colour of the areas covered with plant
growth changes to green. When winter comes on,
the green colour gradually gives place to grey or
brown.

It has been suggested by Arrhenius that the soil
in the dark areas is saturated with soluble salts, like
the alkali flats or salt pans that are found in some
of the desert areas of the Earth. These salts are
hygroscopic and absorb moisture from the air when
the melting of the pole-cap is in progress; the
darkish mud formed when the salts deliquesce
would account for the change in the appearance
of the dark areas. He suggests that in this way
the changing tints could perhaps be accounted for
in the absence of vegetation. The suggestion does
not seem to provide a satisfactory explanation of
the sequence of colour changes recorded by ex-
perienced observers of Mars.

The irregular changes in appearance can be

MARS THE PLANET OF SPENT LIFE 207

attributed to modifications from year to year in the
growth of vegetation, possibly produced by localised
climatic variations from year to year. A particular
region of the planet may receive one year a supply
of moisture that is fully adequate for the scanty
needs of the Martian vegetation; but another year,
the supply over the same region may be insufficient
for these needs and, because of the drought, there
will be a more or less complete failure in the growth
of vegetation. It is not to be expected that on
Mars, any more than on the Earth, the climatic
conditions at any particular place will be exactly
the same each year.

As we have mentioned, the colour of the surface
of Mars provides sure evidence of the presence of
free oxygen, at any rate in the past. The presence
of free oxygen almost certainly demands the
existence of vegetation. Combining this argument
with the evidence from the changes that occur on
the surface, we may conclude that it is almost
certain that there is some form of vegetation on
Mars.

We cannot say whether animal life, and in
particular whether higher forms of life, can exist on
Mars. The small amount of oxygen on Mars seerris
to make this improbable, though we know so little
about life that the possibility cannot be entirely
excluded. But the question whether or not such
forms of life exist on Mars at the present time is
surely of minor significance in comparison with the
very strong evidence that life of some kind is to be
found there. We started out on our survey of the
bodies of the solar system not knowing what we

PLATE 14

THE PLANET MARS

The upper portion of plate shows photographs of Mars
taken on 1939, July 20 (upper left); July 23 (upper
right) ; August 1 1 (lower left) ; August 3 1 (lower right) .
These photographs were obtained by Dr. JefFers with
the 36-inch refractor of the Lick Observatory, through
a yellow screen, so that the contrast in the photographs
is similar to that seen in visual observation of Mars.
The bright cap at the south (upper) pole; the dark areas
(regions of vegetation) ; and the light areas (desert
regions) are well shown. The photographs show also
finer markings which some observers have interpreted
as artificial canals.

The lower portion of the plate provides direct
evidence that the atmosphere of Mars is of considerable
depth. Images of the planet in ultra-violet and infra-
red light are shown; below these the opposite halves of
the two images have been juxtaposed to show that the
ultra-violet image is the larger. The infra-red image,
which shows the surface details, represents the solid
globe of Mars; the ultra-violet image, in which no
surface detail is visible, represents the solid globe sur-
rounded by the atmospheric shell. The difference in
size of the images indicates that the atmosphere of
Mars has a depth of at least 50 miles.

PLATE 15

THE SPIRAL NEBULA, MESSIER 101, IN THE CON-
STELLATION OF THE GREAT BEAR

The objects termed spiral nebulae are stellar universes,
lying outside our stellar universe, with which they are
closely comparable in size and mass. The system,
Messier 101, is seen broadside on, so that its spiral
structure is clearly seen. Like our own stellar universe,
it is slowly rotating in space.

If our own universe could be viewed broadside on
from a great distance, it would appear generally similar
to this system. The condensations of stars in the spiral
arms correspond to the star clouds in the Milky Way.
Patches of bright nebulosity glowing gaseous matter
and dark patches, caused by absorption of light by fine
dust, may be seen; similar features are widespread
throughout the Milky Way regions of our own Universe.

The distance of this system is about i J million light-
years.

Photograph by Dr. Ritchey with the Go-inch reflector
of the Mount Wilson Observatory, 1910, March 11.
Exposure 7^ hours.

2IO LIFE ON OTHER WORLDS

should find but with the expectation that if con-
ditions were suitable for life on any world, life
would somehow have come into existence there.
Our quest for conditions suitable for life to exist
was unsuccessful until we came to Mars; on one
world after another the conditions were found to be
such that we could say with reasonable certainty
that life could not possibly exist. Finally we came
to Mars, when at length we found a world where
the conditions were such that the possibility of life
of some sort existing could not be excluded. And
there we find clear evidence of changes taking place
which we can only attribute to the growth of
vegetation. If this conclusion is accepted, it follows
that life does not occur as the result of a special
act of creation or because of some unique accident,
but that it is the result of the occurrence of definite
processes; given the suitable conditions, these
processes will inevitably lead to the development of
life.

But Mars, though the home of life, is a dying
world. It has lost most of its atmosphere; it has
lost most of its moisture. It may in the past ages
have been the home of animal, and conceivably of
intelligent, life. It does not seem that the present
scanty supply of oxygen can be nearly adequate to
maintain such life. Animals need oxygen to supply
energy, through the process of combustion, which
enables them to maintain the vital processes.
Evolution may secure adaptation to gradually
changing conditions but, with a progressively
lessening supply of oxygen, there must come a time
when adaptation can do no more and the moulding

MARS THE PLANET OF SPENT LIFE 211

powers of evolution succumb before an unequal
struggle. So it seems to me that we must look upon
Mars as a planet of spent life. Such vegetation as
now continues to maintain a precarious existence
must be doomed to extinction in a time which,
geologically, is not remote.

In Venus we saw a world where conditions are
probably not greatly different from those that
existed on the Earth many millions of years ago.
In Mars, on the other hand, we see a world where
conditions now exist which resemble those that will
probably prevail on our Earth many millions of
years hence, when much of our present atmosphere
will have been lost.

CHAPTER IX

THE ORIGIN OF
THE SOLAR SYSTEM

WE have now concluded our survey of the solar
system. With the significant exception of Mars,
the search for any evidences of life has given a
negative result. We have necessarily had to rely
to a considerable extent upon the inferences that
can be drawn from the general conditions prevailing
on the other worlds that we have considered. The
only world on which we might expect to see some
direct evidence of animal life, if it existed, is the
Moon. The surfaces of Venus and the major
planets are permanently hidden from us by clouds
and it would not be possible to obtain any direct
evidence even of vegetation on these worlds, if such
existed. The general considerations about the
nature of life have served as a guide and our
conclusions should be reasonably well established.
The discussion of the conditions prevailing else-
where in the solar systems has provided us with
information that will be useful in trying to assess
the probability that life may exist elsewhere in the
universe.

We have so far dealt only with the family of
our Sun, which is merely an average star, one
amongst the many stars, numbering a hundred
thousand millions or so, in our stellar universe.
And that universe in turn is merely one amongst
many millions of more or less similar island

212

THE ORIGIN OF THE SOLAR SYSTEM 213

universes each a gigantic system scattered
through space.

The question that naturally suggests itself is:
What is the chance that life exists on some of the
planets belonging to one or other of these innumer-
able stars in our own universe or in some other
universe ? This is a difficult question to answer,
because if such planetary systems exist, they are
far beyond our range of vision. If the nearest
known star, twenty-five million million miles away,
had a planet belonging to it of the size of Jupiter
which is much the largest of the planets in our solar
system we should not be able to see it. We must
give up hope, therefore, of ever being able to see
any of the planets that may belong to other stars.
To learn anything about the conditions prevailing
on such worlds is accordingly entirely out of the
question.

But we can perhaps obtain some guidance from
general considerations. If we can find out how the
solar system came into being we shall possibly be
able to judge what likelihood there is that other
stars may have families of planets. For the solar
system has certainly not come into existence as the
result of chance. It is not an accidental collection
of bodies : there are too many regularities in the
system. Let us look briefly at some of these
regularities.

In the first place, all the planets, including some
1,500 minor planets, small bodies whose orbits lie
between the orbits of Mars and Jupiter, revolve
round the Sun in the same direction and their orbits
lie nearly in the same plane. The inclinations

214 LIFE ON OTHER WORLDS

of the planes of the orbits of the planets to the
ecliptic, the plane of the orbit of the Earth, are
mostly only a few degrees. With the exceptions of
Pluto and of some of the asteroids, the largest
inclination is 7 for the orbit of Mercury. The incli-
nation of the orbit of Pluto is 17 and some of the
minor planets have still large inclinations, but the
orbits of these bodies may have suffered consider-
able perturbations. The orbits, again with the
exception of those of Pluto and of some of the
minor planets, are nearly circular in shape. The
Sun rotates in the direction in which the planets
revolve and the orbits of the planets lie nearly in
the plane of the Sun's equator; the planets also
rotate in the same direction as the Sun. The
orbits of the satellites, with a few exceptions, are
nearly circular and lie nearly in the plane of the
parent planet's equator. The satellites revolve
around the planets in the same direction as that
in which the planets themselves rotate, and their
movements are therefore in the same sense as those
of the planets. We may compare the solar system
with a pack of cards as it comes from the makers,
in which the cards are arranged in suits and each
suit in order of value, rather than with a pack
which has been used and shuffled. The regularities
demand an explanation, but to find an explanation
of how a system such as the solar system could have
come into being has proved to be the most difficult
of all the problems of cosmogony.

The oldest hypothesis that we need mention
seems first to have been suggested by that versatile
Swedish scientist and theologian, Emanuel Sweden-

THE ORIGIN OF THE SOLAR SYSTEM 215

borg and, somewhat later but probably indepen-
dently, by an Englishman, Thomas Wright, of
Durham. It was adopted by the German philo-
sopher, Immanuel Kant, in his essay on the general
history and theory of the heavens, published in
1755. Kant started from the assumption that the
material, which now forms the Sun, the planets and
their satellites, was formerly a diffuse nebula. He
supposed that the different attracting powers of the
various elements of this nebula would cause a loss
of homogeneity; the heavier elements would tend
to fall to the centre but their fall would be opposed
by the tendency of the gas to expand. He im-
agined that in this process lateral movements would
be set up and that from these movements there
would ensue a rotation of the whole mass. Kant
was in error in supposing that rotation could be
started in this way; this assumption is in direct
opposition to one of the general principles of
mechanics, the principle of conservation of angular
momentum.

As this principle is of great importance in dis-
cussing the origin of the solar system, a few words
in explanation will not be out of place. Without
attempting any precise or strictly logical definition,
the angular momentum of a body is a measure of the
total quantity of rotational motion that it contains.
Suppose, for instance, that the body is spinning
around an axis, like the Earth. Imagine it to be
divided up into equal parts, each weighing one
pound. The angular momentum of any of these
parts can be measured by the product of its velocity
and its distance from the axis. The angular mom-

2l6 LIFE ON OTHER WORLDS

en turn of the whole body is the sum of the ahgular
momentum of all the parts. Consider, now, the
Earth, which rotates on its axis once in the course
of a day. If the Earth were to expand, the dis-
tance of each part from the axis would be increased.
As the total angular momentum must remain
constant when no external forces are acting on the
body, the velocity of each part must decrease to
compensate for the increased distance from the
axis; in other words, the rotation must be slowed
down. The day would therefore become longer.
Kant, in his theory, supposed that a general
rotation developed out of local rotations. But
these local rotations must have been some in one
direction and some in another and the net effect
was nil, for there was no rotation and no angular
momentum to begin with. The total angular
momentum must therefore have remained zero,
so that the local rotations could never give rise to
a general rotation.

Kant's theory was revived in a modified and
more scientific form by the great French geometer,
Pierre Simon de Laplace, the Newton of France,
in his Exposition du Systeme du Monde, published in
1796. Laplace avoided the difficulty of explaining
how the rotation had been started by postulating
that the original diffuse nebula was itself in slow
rotation. As the nebula cooled it gradually shrank
and became more dense; at the same time, its rate
of rotation necessarily increased, in order to con-
serve its angular momentum. With increase in the
rate of rotation the centrifugal force at the equator
increased until at length it became equal to the

THE ORIGIN OF THE SOLAR SYSTEM 2iy

force t)f gravity. When this occurred, Laplace
supposed that a ring of matter split off round the
equator. The main mass went on contracting still
further and in due course another ring would be
shed and so on. The successive rings somehow
condensed to form the planets; the planets them-
selves passed then through a similar type of
evolution, contracting and throwing off rings, which
in turn formed the satellites.

Laplace's exposition, though clothed in mathe-
matical language, was semi-popular and purely
qualitative. He did not discuss the problem in a
mathematical or quantitative way to prove that the
evolution of the primitive nebula must follow the
course he suggested. The theory is attractive
because it readily accounts for the motions of the
planets and of their satellites being in the same
general direction and nearly in the same plane, and
for a long time it was accepted as providing a
satisfactory explanation of the main features of the
solar system. Unfortunately for the theory, how-
ever, there are objections to it that are fatal.

In the first place, Laplace assumed that a ring
of matter would be thrown off around the equator
of the contracting mass and that this ring would
coalesce into a single body. He offered no proof
of this assumption. On the other hand, it was
proved by strict mathematical arguments by Clerk
Maxwell in 1859 that coalescence into a single body
could not occur, but that a ring of small bodies
moving around the parent mass in similar orbits
would result and that such a ring would form a
stable configuration. The rings of Saturn provide

2l8 LIFE ON OTHER WORLDS

an example of such a stable system consisting of a
large aggregation of small discrete bodies.

This objection is in itself sufficient to disprove
Laplace's hypothesis in the form in which it ap-
pears in the Systeme du Monde. But there is a further
and equally serious objection. When we evaluate
the angular momenta of the different members
Sun, planets and satellites of the solar system,
we find that Jupiter contributes more than half the
total amount and that the four major planets
between them account for about 98 per cent, of
the whole. The remaining 2 per cent, is provided
almost entirely by the rotation of the Sun itself,
the four inner planets Mercury, Venus, the
Earth and Mars together contributing only o-i
per cent, of the total. Thus almost the whole of
the angular momentum of the system is contributed
by the four major planets, which have less than
one-seven-hundredth of the total mass of the system.
We know by the principles of mechanics that the
angular momentum of the solar system at the pre-
sent time must be equal to the angular momentum
of the primitive nebulous mass, from which it is
supposed to have been formed. The problem is to
explain how nearly the whole of the angular momen-
tum was captured by such a small portion of the
whole system.

We can look at the problem in another way.
Suppose the Sun and all the planets were united to
form a spherical nebulous mass of uniform density
extending to the orbit of Pluto. We suppose that
this mass is rotating with the period of revolution of
Pluto, for a condensation within the mass must be

THE ORIGIN OF THE SOLAR SYSTEM 2IQ

assumed to have revolved with the same period as
the mass itself. The angular momentum of such a
system is found to be less than one- two-hundredth
part of the actual angular momentum of the solar
system. Since, in the process of contraction sup-
posed by Laplace to have occurred, angular momen-
tum could neither have been lost nor gained, it is
certain that the present distribution of angular
momentum could not have been produced in the
way that Laplace supposed. The only possibility
of accounting for the present distribution of angular
momentum is to suppose that there was some
process by which the angular momentum of the
major planets was imparted to them from outside
the Sun or primitive nebula by some force of a
transitory nature. The origin of the solar system
is not to be explained by the gradual cumulative
action of internal forces; an explanation must be
sought in the swift catastrophic action of forces
from outside.

Attempts have accordingly been made to account
for the origin of the system by supposing that at
some time in its past history another star passed
very close to the Sun, narrowly avoiding an actual
collision. Let us consider what sequence of events
would be likely to occur. As the other star ap-
proached the Sun, its gravitational attraction would
distort the Sun by raising a tidal protuberance on
it; the Sun would raise, in turn, a tidal protuber-
ance on the approaching star. As the distance
between the Sun and the star became smaller the
tides raised on the two bodies would gradually
increase more and more. Suppose the passing

22O LIFE ON OTHER WORLDS

star did not pass nearer to the Sun than a couple of
million miles or so, the tide raised on the Sun would
reach its maximum height when the star was at its
nearest to the Sun and would then fall back as the
star moved away, causing an oscillation or pulsa-
tion in the Sun which would gradually be damped
out by friction.

Suppose, however, that the star passed so close
to the Sun that its attraction on the nearest portion
of the Sun's surface was greater than the force
of the Sun's gravity; material would then be drawn
out from the top of the tidal protuberance by the
gravitational attraction of the passing star. The
ejection of matter from the Sun would occur
slowly at first but at a gradually increasing rate as
the star drew nearer. The ejection would be most
rapid when the star was at its nearest to the Sun
and would become progressively less rapid as the
star moved away, until at length it ceased alto-
gether. Some of the material torn from the Sun
would probably be captured by the passing star
and some would probably escape into space.
But some would remain under the control of the
Sun's gravitation and it is supposed that the planets
ultimately condensed out of this material.

These assumptions have formed the basis of
several theories of the origin of the solar system.
Such theories have the advantage over the theory
advocated by Laplace that the angular momen-
tum of the planets can be attributed to the action
of the passing star. The ejected material would
form a jet curved towards the star and the angular
momentum of this jet is derived from the angular

THE ORIGIN OF THE SOLAR SYSTEM 221

momentum of the star. After the encounter, the
ejected material would be moving in the plane of
the path of the star past the Sun and in the same
direction round the Sun. The theories of this
type thus account in a perfectly natural manner for
the orbits of the planets and of their satellites being
nearly in one and the same plane and for the
direction of their revolutions being all the same.
Closer examination is needed before we can say
whether such theories are free from objection when
considered quantitatively; they seem to be satis-
factory when we consider them merely qualita-
tively.

The first theory of this nature was proposed
about forty years ago by Professors T. C. Chamberlin
and F. R. Moulton and is known as the " planet-
esimal theory." Observations of the Sun show
that its surface is in a state of continual disturbance
and that, from time to time, incandescent material
is thrown up with great force from the surface to
considerable heights. Chamberlin and Moulton
supposed that such disturbances, under the action
of the gravitational pull of the passing star, resulted
in huge eruptive bolts of matter being ejected from
the Sun with great violence. From the opposite
side of the Sun, where the attraction of the passing
star was much smaller, another series 01 bolts,
but of smaller size, were ejected. They supposed
that the larger bolts ultimately gave rise to the
major planets and the smaller bolts to the terres-
trial planets.

The material that was shot out from the Sun by
the eruptions, enormously intensified by the tidal

222 LIFE ON OTHER WORLDS

forces produced by the approach of the other star,
would rapidly cool. Before long it would liquefy,
to form a large number of separate small bodies,
each moving around the Sun in its own orbit
almost independently like a planet. Soon after-
wards these would solidify. The separate small
bodies were called planetesimals. Their distribution
would not be uniform; the densest portions of the
bolts would give rise to close aggregations or
swarms of particles. It was supposed that these
collected into solid cores, which became the nuclei
of the planet. As these nuclei moved through the
swarms of planetesimals, they gradually gathered
them in, one by one, through the action of their
gravitation, until at length the neighbourhood of
the Sun had been swept clean. The numerous
minor planets or asteroids arose from a mass of
material which was deficient in any large nucleus,
so that there was nothing to sweep them up.
The satellites of the planets are supposed to have
been formed from smaller secondary nuclei, which
were saved from being drawn in to the larger nuclei
by possessing a sufficient velocity to enable them to
revolve around the parent planetary nucleus, just
in the same way as it was the motion of these
nuclei that kept them from falling into the Sun.

Some of the material that had been drawn out
from the Sun would, of course, fall back upon it,
as the result of the gravitational pull of the Sun.
This material had acquired angular momentum, in
the same direction as the planets, from the passing
star. When it fell back into the Sun, its angular
momentum was communicated to the Sun, which

THE ORIGIN OF THE SOLAR SYSTEM 223

in this way acquired a rotation in the same direction
as that in which the planets revolve round the Sun.
A similar explanation can be used to account for
the axial rotations of the planets.

At the time that the planetesimal theory was
put forward, very little was known about the
propulsional forces that were responsible for the
ejection of material from the Sun, seen in some of
the eruptive prominences. It is now known that
these forces arise from the pressure of the Sun's
radiation; though this pressure can impart high
velocities to particles of molecular dimensions, it
is not competent to play the important role of a
trigger releasing bolts of matter sufficiently massive
to form the planets. This objection is avoided in a
modified tidal-ejection theory proposed by Sir
James Jeans.

Jeans showed that the effect produced by the
passage of the star past the Sun would depend
upon the physical nature of the Sun. If the Sun
were of uniform density throughout and incom-
pressible in other words, very much like a solid
body the effect would be to disrupt it into pieces
comparable in size and mass with the parent
body. If, on the other hand, the density of the
Sun increases rapidly inwards, the disrupted frag-
ments are small, and the mass of the Sun would
not have been much affected. The ejected matter
would have come from the outer layers of relatively
low density. The density of the Sun must increase
rapidly from the surface inwards towards the
centre and the conditions therefore approximated
to the second case. This being so, the passing star

PLATE i 6

SPIRAL NEBULA IN THE CONSTELLATION OF
BERENICE'S HAIR

The object reproduced on this plate is a spiral nebula
seen almost exactly edge on. It illustrates the aptness
of Herschel's description of our stellar universe as being
a much-flattened system, shaped like a grindstone. The
bright central nucleus will be noted, as well as the dark
absorbing matter scattered through the median plane.
Such absorbing matter, fine dust, is widely scattered
through the central regions of the Milky Way, causing
the Milky Way through a great part of its extent to
appear to have two branches; the central portion is
not seen through the fog of dust.

If our own universe were viewed edgewise-on from a
great distance, it would appear like this nebula. The
Sun has a position about midway between the central
nucleus and the outer edge.

The distance of this system is about 6 J million light-
years.

Photograph by Dr. Ritchey, with the Go-inch reflector
of the Mount Wilson Observatory, 1910, March 6-7.
Exposure 5 hours.

PLATE 17

SPIRAL NEBULA, MESSIER 81, IN THE CONSTEL-
LATION OF THE GREAT BEAR

The spiral system is seen obliquely. Other systems
can be seen at all angles of view, from broadside-on to
edgewise-on, establishing the general similarity of the
objects shown in the two preceding plates, though at
first sight they would seem to have no points of re-
semblance.

The nebula. Messier 81, has a very bright nucleus.
Aggregations of stars, in the form of star clouds, can be
seen in the outer extensions of the spiral arms ; patches
of dark nebulosity, caused by obscuring dusty matter,
and of bright gaseous nebulosity are clearly visible in
the inner regions.

The distance of this system is about 2^ million light-
years.

Photograph by Dr. Ritchey, with the Go-inch reflector
of the Mount Wilson Observatory, 1910, February 5.
Exposure 4^ hours.

226 LIFE ON OTHER WORLDS

would give rise to a tidal protuberance, from whose
tip matter would be drawn out in the form of an
elongated jet or filament. Jeans showed that
unless this filament were of uniform density, which
would have been extremely unlikely, there would
be a tendency for the material to condense around
any portions of greater density. The filament
would therefore break up under its own gravitation
and form a number of separate bodies which, after
the disturbing star had gone its way, would move
in orbits around the Sun. As the outflow of
matter would be greatest when the star was at its
nearest, the filament would be thickest near its
middle, tapering off towards both ends. It is
accordingly to be expected that the largest planets
will be found in the median range of distance, with
smaller planets at the two extremes of the range.
This is exactly what we find in the solar system,
Jupiter and Saturn being the largest and most
massive planets whilst Mercury and Pluto are the
smallest and least massive.

As in the planetesimal theory, there would be
a portion of the ejected material which would not
be drawn in by gravitational attraction to any of the
planetary aggregations. This would form a mix-
ture of gaseous matter and solid particles. On
either this theory or the planetesimal theory the
motion of the planet through this material, which
would act as a resisting medium, would make their
orbits less elliptical and more nearly circular.
At the same time the planets would gradually
sweep up this material. The nearly circular shape
of the orbits of the planets can thus be explained.

THE ORIGIN OF THE SOLAR SYSTEM 227

It is supposed that the satellites of the planets
were formed in an analogous manner. The pass-
ing star or the Sun, or possibly both, would have
produced tides on the primitive planets, causing
filaments of matter to be ejected which condensed
into satellites. Just as some of the ejected material
after falling back on to the Sun is supposed to
have set it into rotation, so some of the material
ejected from the planets is supposed to have fallen
back on to their surface and to have set them into
rotation.

But this raises a difficulty. To explain the rapid
rotation of Jupiter it is necessary to suppose, as
was shown by Dr. H. Jeffreys, that the material
falling back on to its surface had a mass about
one-fifteenth of the mass of Jupiter, or about
400 times the mass of all its satellites. It is not
possible to believe that so small a proportion of
the ejected material would condense to form
satellites and that the bulk of the material would
fall back to the surface of the planet. There are
similar difficulties in accounting for the rotations of
the other major planets Saturn, Uranus and
Neptune and of Mars.

Dr. Jeffreys showed that this difficulty could be
avoided if somewhat more specialised conditions
for the approach of the star to the Sun are assumed.
Instead of supposing that the star passed the Sun
at a distance of a couple of million miles or so,
he supposed that there was an actual collision.
The picture of what happened is then somewhat as
follows. As the Sun and star drew near to one
another, their velocities rapidly increased, under

228 LIFE ON OTHER WORLDS

the influence of their rapidly increasing mutual
gravitation, until at the instant of collision their
relative velocity attained the enormous value of
several hundred miles a second. The collision was
neither a direct collision nor a grazing one, but
sufficiently tangential for the heavy central cores
of the two bodies to miss one another.

Whilst the star was drawing rapidly near to the
Sun, enormous tidal distortions would be raised
both on the Sun and on the star, from which
ejection of matter would begin to occur shortly
before the actual collision took place. When the
collision occurred, the outer layers of the two
bodies within the area of impact would intermingle,
whilst the heavy central portion of the star would
pursue its headlong way and, swinging round in a
hyperbolic path, would recede away into space.

The intermingling layer, greatly compressed by
the impact between the two bodies and intensely
heated by friction, would be thrown into an ex-
tremely turbulent state, with rapid rotation caused
by the shearing motion of the two bodies. As the
star moved away, this rapidly rotating layer would
be stretched out all the way from the Sun to the
star and would form a ribbon or filament, which
would take the place of the tidal filament in the
theory of Jeans. The whole of this stupendous
catastrophe, leaving its permanent mark on the Sun
in its system of planets, would be over in about
an hour.

Jeffreys showed that this hypothesis of an actual
collision would account satisfactorily, not merely
qualitatively but also quantitatively, for the rates

THE ORIGIN OF THE SOLAR SYSTEM 22Q

of rotation of the Sun and planets and for the total
mass of material forming the planets.

But in removing one difficulty we have intro-
duced others of a different nature, though equally
serious. The mass of the Sun was not much
affected by the collision, which merely tore off a
part of its outer layer of low density, leaving the
heavy central core unaffected. The luminosity
and size of a dwarf star like the Sun is determined
primarily by its mass and it therefore follows that
at the time of the collision the Sun was practically
in its present state and, in particular, its size was
near about what it now is. The temperature of
the highly compressed and rapidly rotating layer
from which the planets are supposed to have been
formed must have been of the order of ten million
degrees. At so high a temperature, the average
velocities of the atoms are so great that the attrac-
tion of the filament could not hold the material
together. It would rapidly diffuse away into space.
It is difficult to comprehend how bodies of the size
of the major planets could have condensed out of
such a filament.

Suppose, however, for the sake of further argu-
ment that this difficulty could be overcome. We
still have to face an even more serious difficulty,
whether we suppose that there was an actual
collision between the Sun and the star or merely a
close approach. This difficulty is concerned with
the distribution of angular momentum in the solar
system. If the amount of the angular momentum
per ton of matter is calculated for each of the
planets, we find that there is a steady increase

230 LIFE ON OTHER WORLDS

outwards from the Sun. If we take the angular
momentum per ton for the Earth as the unit in
which to express the values of the angular momen-
ta per ton of the other planets, the values increase
from 0-6 for Mercury to 6-1 for Pluto. Now
compare these values with the angular momentum
per ton of the star. Suppose that the star was of
about the same size and mass as the Sun and that
at its nearest approach it was about 1,500,000
miles from the centre of the Sun. Its angular
momentum per ton can be shown to be about 0-25
in the same unit. If, on the other hand, the star
had actually collided with the Sun, the angular
momentum per ton is found to be much smaller
even than this value.

The average angular momentum per ton of
the planets is about ten times greater than the
average angular momentum per ton of the in-
truding star, if there was no collision. If a collision
had occurred, the discrepancy becomes even
greater. The difficulty is to explain how so much
angular momentum could have been put into the
matter ejected from the Sun.

The difficulty involved can perhaps be better
realised in this way. The angular momentum
per ton depends upon two quantities: the distance
of the matter from the Sun and its velocity at right
angles to the direction to the Sun. If the matter is
moving directly towards or away from the Sun, it
has no angular momentum about the Sun. Now
the velocity of the star was such that it flew in a
hyperbolic orbit away into space; if any of the
ejected matter had moved in the path of the star,

THE ORIGIN OF THE SOLAR SYSTEM 231

with the same velocity as the star, it would have
done the same and would not have condensed into
planets. How then has it happened that the
matter that did condense into planets has acquired
angular momentum (per ton) so much greater
than the star and yet has not escaped into space
but has remained bound by the Sun's gravitation,
whilst the star was able to escape and has not
remained held by the Sun's gravitation to form a
double star ? We must remember that the matter,
which subsequently condensed into the planets, is
supposed to have been drawn out initially in a
curved jet, stretching from the Sun towards the
passing star. To have the requisite angular mo-
mentum we need the matter to be moving at right
angles to its direction to the Sun, instead of nearly
in a radial direction.

A mathematical discussion of the problem is
needed for the difficulty to be fully appreciated,
and such a discussion would be beyond the scope
of the present book. The discussion shows that it
is impossible to account for the angular momentum
per ton of matter of the planets being so much in
excess of the angular momentum per ton of matter
of the star, whatever assumption is made about
the speed and direction of approach of the star.
Attractive as these theories of the origin of the solar
system seemed at first sight, we are reluctantly
compelled to discard them as unworkable. The
hypothesis of an actual collision, introduced to
provide a satisfactory explanation of the rapid
rotations of some of the planets, proves to raise
greater difficulty than the hypothesis of a close

232 LIFE ON OTHER WORLDS

approach, when we consider the angular momenta
of the planets per ton of matter.

In order to avoid this very serious difficulty, a
further modification of the theory has been at-
tempted. Attention may here be drawn to the
circumstances that each new objection raised
against any theory of the origin of the solar system
has to be overcome by the introduction of some new
additional assumption, making the theory in itself
less probable. But to account for the solar system,
as it now exists, is so beset with difficulties that we
cannot reject any theory, however improbable it
may seem, if it gives a plausible explanation of
how such a system could come into existence and
if it encounters no serious objections. The solar
system must have had an origin; if we cannot
account for it except by the introduction of many
special and somewhat artificial hypotheses, we shall
have to conclude that the probability of other stars
having systems of planets is very small.

In the new modifications of the theory, suggested
by Prof. H. N. Russell and developed by Dr.
Lyttleton, it is supposed that before the encounter
with the passing star, the Sun was a twin star. This
is not in itself a very improbable assumption, for
it is known that a considerable proportion of the
stars, perhaps one in five, is a twin system. It is
suggested that the Sun's companion was a good
deal smaller than the Sun and that its distance
from the Sun was comparable with the distance of
the major planets. The passing star, which was
much more massive, is supposed to have collided
with the companion to the Sun, breaking it into

THE ORIGIN OF THE SOLAR SYSTEM 233

fragments and knocking it out of its orbit. The
assumption that, if two stars collided, one of them
would be broken up into several fragments of
comparable size is not capable of verification by
mathematical discussion and may not be correct.
This is a weakness of the theory, which we must
tentatively pass over.

The passing star is assumed to have made an
almost central collision with the companion and
to have carried away with it most of the fragments,
leaving merely the debris of the collision as the
material which remained associated with the Sun
to form the planets. Moreover, unless the plane of
the orbit of the original companion about the Sun
and the path of the intruding star were nearly
parallel, the orbits of the fragments that remained
after the collision would not be nearly in one
plane and could not therefore lead to a system like
our solar system. The theory thus abounds with
special assumptions. But it gets round the two
difficulties of the previous theories, the difficulty of
accounting for the rapid rotations of the planets
and the difficulty of accounting for their large
angular momenta. The rotations of the planets
are readily attributed to the collision and the
angular momentum of the planets was derived from
the shattered companion; it was there initially and
the collision merely involved redistribution of the
initial angular momentum amongst the fragments.

There has been a good deal of controversy about
whether this theory of the origin of the solar
system can be made to work. Very special con-
ditions are required for the intruding star to be

234 LIFE ON OTHER WORLDS

able to get away, carrying along with it the bulk of
the disrupted companion of the Sun, whilst leaving
behind sufficient material to form the planets.
A complicating factor is that mathematics is not
able to trace out in detail exactly what would hap-
pen. It seems not impossible, however, that with
specially arranged conditions, in which not very
much latitude can be allowed in any particular, the
solar system might have been produced by a col-
lision between an intruding star and a companion
to the Sun. But so many special assumptions are
involved that it has been remarked that the solar
system had a very narrow escape from never
coming into existence.

Though this is not a valid argument against the
theory, there are many who are disinclined to
accept a theory which seems so inherently im-
probable. This is not a logical attitude. Unless
we can find a more plausible origin for the solar
system, we must provisionally accept any theory
that can be made to work. If the theory is im-
probable we are able, at any rate, to infer something
about the likelihood that other stars may have
systems of planets circulating about them. This
will give us a clue whether life is likely to be wide-
spread throughout the universe or to occur only
exceptionally.

On any theory that requires either a near
approach of a star to the Sun or an actual collision,
it must follow that planetary systems are of very
exceptional occurrence. The stars are so thinly
scattered through space that the chance of a close
approach of any two stars is extremely small.

THE ORIGIN OF THE SOLAR SYSTEM 235

The nearest known star is 25 million million miles
away; it is obvious that, with stars at this sort of
distance apart, it will be a very exceptional thing
for one star to pass near or actually to collide with
another. As it is difficult to conceive of distances
measured by millions of millions of miles, we can
get a picture that the mind can more readily grasp
in this way. Suppose we have a hollow globe the
size of the Earth, 8,000 miles in diameter, and that
we put half a dozen tennis balls inside it and allow
them to fly about in any direction, rebounding
from the wall when they hit it. The chance that
two of these balls will collide is about equal to the
chance that two stars will come into collision.

The chance that two stars will actually collide
was computed by Sir James Jeans. He found that
a given star will be likely to meet with an actual
collision only once in 600,000 million million
(6 X io 17 ) years. The chance of two stars approach-
ing each other sufficiently closely, without an actual
collision, to give rise to the ejection of a tidal
filament is somewhat, but not much, greater.

From these figures we can make an estimate of
the number of stars that are likely to have experi-
enced a close approach to another star. As a
rough, but sufficiently close estimate, we will sup-
pose that any given star makes a close approach to
another star on the average once in 500,000
billion 1 (5 X io 17 ) years. The average age of the
stars is believed to be greater than 10,000 million
(io 10 ) years. It follows that not more than one
star in fifty million is likely to have collided, or

1 Billion is used to denote one million million.

236 LIFE ON OTHER WORLDS

nearly to have collided, with another star in the
whole of its life. In our own stellar universe the
number of stars is of the order of 100,000 million.
These stars are not distributed uniformly; there are
localised aggregations or clusterings where the star
density is above the average and regions where the
stars are relatively sparse. If we assume that the
star density in the vicinity of the Sun is sufficiently
representative of the system as a whole, it will
follow that in our own stellar universe there are not
likely to be more than several hundred stars that, at
some time or other in the course of their life, have
had a close approach to another star, which ap-
proach has resulted in the ejection of a tidal filament
of matter.

No great exactitude can be claimed for these
figures. But, after making adequate allowance for
all possible uncertainties in the data, there is no
escape from the conclusion that the number of
stars in our stellar universe which in the course of
their lifetime have made a close approach to
another star is very small a mere few hundred.
Until recently it was thought that each such close
approach would result in the formation of a
planetary system; but now, as we have already
mentioned, it is realised that this is far from being
so and that it is only under very specialised assump-
tions that the theory could be made to work.
The close approach will lead to the ejection of a
stream of matter from the great tidal bulge drawn
up on the star; but this matter will normally be
dissipated away into space and will not condense
into discrete planets, held bound to the parent Sun

THE ORIGIN OF THE SOLAR SYSTEM 237

by the force of gravitation. To ensure this, we
have been compelled to assume that the Sun was
initially a double star; and not only so but, in order
to make the theory workable, it becomes necessary
to impose special conditions on the nature of the
encounter between the two stars. There are
limitations to the direction and velocity of approach
of the intruding star which are necessary if a system
like the solar system is to result from the encounter.
The effect of these limitations is very seriously to
reduce the chance that a close encounter between
two stars will give birth to a system of planets.
Precise estimation of the probability is not possible
but it seems likely that out of several hundred
encounters, each sufficiently close to result in the
ejection of matter from the tidal protuberance,
there can at the most be very few that give rise to
planetary systems. Our solar system appears,
therefore, to be a system that is nearly, though per-
haps not quite, unique in our stellar universe.

There is one possible way of escape, however,
from this conclusion. For the estimate of the chance
of a close approach of two stars has been based
upon the observed average distance apart of one
star from another and it is implicitly assumed that
this distance has remained the same throughout the
lifetime of the stars. Modern investigations sug-
gest, on the contrary, that the validity of this assump-
tion is doubtful: that, in fact, the stars may have
been very much closer together when the Earth
and the other planets were born than they are at
the present time.

It would lead us too far afield to go fully into the

238 LIFE ON OTHER WORLDS

reasons for this conjecture. There are, however,
two main lines of approach. The first line of
approach is through the consideration of the prob-
lem of the source of stellar energy. The age of the
Earth is somewhere about 3,000 million years;
the Sun must be at least as old as the Earth. And
for this period of time it has been lavishly pouring
out energy in the form of heat and light into space.
Where does this energy come from ? This is a
problem that has occupied astronomers for many
years. Lord Kelvin suggested that the Sun was
contracting under its own gravitation; the process
of contraction would release energy, which the
Sun radiates away into space. It is now known that
energy provided in this way would not maintain
the Sun's radiation for more than about 20 million
years. When radioactivity was discovered, it was
thought that this might provide an explanation;
but the supply of energy that could be provided
again proved hopelessly inadequate.

Meanwhile, more precise information about the
age of the Earth was being obtained and it was
realised that the only processes that could main-
tain the Sun's radiation for thousands of millions
of years must be of an atomic nature. There
were two main alternatives. The energy might be
derived from the actual annihilation of matter or
from the building-up of heavier atoms from atoms
of hydrogen. The modern conception of matter is
that it is built up of elementary particles, protons
and electrons, of positive and negative electricity.
If we could bring a proton and an electron together,
so that their charges coalesced and neutralised one

THE ORIGIN OF THE SOLAR SYSTEM 239

another, both particles would disappear and a
splash of energy would result. The amount of
energy locked up in the atom, which might con-
ceivably be released in this way, is prodigious.
From one ounce of coal we should obtain sufficient
energy to run engines of a total horse-power of
100,000 h.p. for one year; in other words, the fuel
requirements of a large generating station could be
satisfied by a fuel supply of one ounce of coal a
year. If annihilation of matter provided the
source of the energy of the stars, they would be
able to maintain their output of energy without
very serious diminution for millions of millions of
years. For a time, indeed, various considerations,
into the details of which it is not necessary to enter
here, led astronomers to believe that the ages of
the stars were, in fact, of the order of millions of
millions of years. It seemed, therefore, that the
process of annihilation of matter must actually be
taking place within the intensely hot interiors of the
stars.

There was a difficulty, however, which could not
easily be resolved. Investigations into the internal
constitution of the stars led to the conclusion that
the temperatures at the centres of the stars were
of the order of from 10 to 20 million degrees.
Theoretical considerations suggested that the anni-
hilation of matter could not occur at temperatures
below some thousands of millions of degrees. The
supposition that annihilation of matter can occur
in the interior of the stars appears, therefore, to be
untenable.

We are consequently thrown back on the alterna-

24O LIFE ON OTHER WORLDS

tive that the energy of the stars is derived from the
building up of heavier atoms from atoms of hydro-
gen. A helium atom can be built up from four
atoms of hydrogen. But the weight of the helium
atom is less than the weight of the four hydrogen
atoms by about i part in 140. From four pounds
of hydrogen we could only obtain about 3 Ib.
15^ oz, of helium. What has happened to the
missing half-ounce ? It is accounted for by the
energy that has been released in the process. We
can suppose also that elements heavier than
helium are built up from hydrogen atoms; the
energy set free is then greater, though not appre-
ciably greater, than in the building up of helium.
The energy obtained by building up heavy atoms
from other lighter atoms, with the exception of
hydrogen, is also relatively small. It is the first
step in the process of building up, the formation of
helium from hydrogen, that provides the bulk of
the energy that can be derived by the atom-
building processes. It is conceivable that, in the
beginning, all matter consisted of the elementary
particles and that the heavy elements have been
built up stage by stage from these particles. The
process can go on until we arrive at the heavy
atoms, such as those of radium, thorium and
ceranium, which are inherently unstable and which
spontaneously disintegrate.

The energy that can be released by atom-build-
ing processes is only about one per cent, of the energy
that can be provided by the annihilation of matter.
The time during which the stars can continue 4 to
radiate is therefore much shorter. The Sun con-

THE ORIGIN OF THE SOLAR SYSTEM 24!

tains at present about one- third part by weight of
hydrogen; the other two- thirds consist of heavier
elements. Whilst the age of the stars can be ex-
tended to several million million years if annihila-
tion of matter takes place, the age is limited to
about 10,000 million years if annihilation does not
occur and atom-building provides the main source
of energy. As the age of the Earth is about 3,000
million years, it would seem that the stars cannot
be much older than the Earth itself, a conclusion
that is somewhat unexpected.

The second line of approach to the question
whether the stars were closer together when the
planets were born than they are now is through
the information that has been obtained about the
expansion of the universe. It has been found by
observation that the various stellar universes, the
so-called spiral or extra-galactic nebulae, are all
receding from each other, with speeds that are
proportional to their distances apart. From which-
ever of these universes observations are made, the
other universes appear to be receding from it, and
the more distant the universe the faster is its velocity
of recession. This has been interpreted as evidence
that the Cosmos as a whole is expanding. Imagine a
rubber balloon being inflated and suppose that on
its surface we have marked a number of ink-dots,
As the balloon expands, each dot will increase its
distance from every other dot and the rate of in-
crease of the distance between any two dots will be
proportional to that distance. This is analogous
to what we observe in the motions of the various
stellar universes.

16

242 LIFE ON OTHER WORLDS

The rate at which these universes are receding
from each other is such that all distances are
doubled in about 1,300 million years. If this rate
of recession had remained constant in past time, it
would follow that the various universes were very
much closer together when the solar system was
formed than they are now. If we go back some-
what farther in time, we should find that they were
all crowded together in a relatively small volume of
space; if our conclusions about the ages of the stars
are correct, it was then that the stars were born.

When we seek to probe backwards in time, we
do not seem to be able to get beyond a time a few
thousand million years ago. At such a time in the
past, we find the various universes congregated
close together in a volume of space much smaller
than they now occupy; at such a time the stars were
born; at such a time the solar system itself was born.
It seems as though time stood still until all sorts of
things began to happen and time began to move.
Exactly what did happen at that time, and why it
happened, we do not know. It has been suggested
that at this epoch, which measures for us the
beginning of time, all the matter in the entire
Cosmos was closely packed together that there
was in the beginning, in fact, one great Atom.
Something then happened that was in the nature of
an explosion; the Cosmos was shattered and frag-
ments were hurtled in all directions through space.
They would continue to move with their initial
velocities and, at any subsequent time, the fastest-
moving fragments would naturally be found to have
moved to the greatest distances. The appearance

THE ORIGIN OF THE SOLAR SYSTEM 243

would be exactly that revealed by observation, of
an expanding universe. At about this same time,
or perhaps shortly afterwards, the stars were born.
In these early ages, when the universe as we know
it was beginning to take shape, the stars must
have been much closer together than they now are.
Collisions and close approaches between stars, or
between dense aggregations of stars, must have been
frequent, and it is possible that in all this turmoil
the birth of planetary systems may have been
widespread.

A way of escape from the impasse into which we
were landed in our attempts to account for the
origin of the solar system may thus be provided. It
had seemed that such a combination of initial con-
ditions was required to enable the system to come
into existence that the system must be almost
unique; it now appears that the conditions under
which the system was born may have been so differ-
ent from those which are now revealed to us by
observation that we cannot draw any certain con-
clusions. Though we are still unable to say in
detail how the solar system originated, we are
probably correct in concluding that it is by no
means so exceptional as we had thought and that
there are likely to be many other stars, in addition
to the Sun, which 'are accompanied by systems of
planets.

CHAPTER X

BEYOND THE SOLAR SYSTEM

IN the preceding chapter we discussed in some
detail the origin of the solar system in the hope that
we might be able to estimate the likelihood that
other stars may have families of planets. We found
that no definite answer was possible because we
could not say with certainty how the solar system
had come into being. It has proved to be one of
the most difficult problems of astronomy. If the
stars have always been distributed in much the
same way as they are at present, we seem to be
forced to the conclusion that the proportion of stars
that have families of planets must be extremely
small. We cannot be certain, of course, that we
may not have overlooked some possibility. There
seems to be no obvious loophole in the arguments,
and I can see very little hope that the solar system
will be explained by means of some event in the
Sun's history that might equally well have happened
to the majority of the stars. Provided that the dis-
tribution of the stars has not greatly changed in the
past, it seems almost certain that very exceptional
circumstances are required to explain the origin of
the solar system.

Such was the position until recent years. The
theory of the expanding universe was then formu-
lated to provide an explanation of the observed
motions of the distant universes, which appear to be
all receding from us, the more distant the universe

244

BEYOND THE SOLAR SYSTEM 245

the greater being the velocity of recession. I use
the word appear advisedly, because the theory of
the expanding universe is based upon a particular
interpretation of observations. If a body which is
sending out radiations is moving towards us, the
radiations are compressed and appear to us to be of
shorter wave-length than they would if we were on
the moving body; if the body is moving away from
us, the radiations that we receive appear to be of
longer wave-length. This change of wave-length
due to motion is exemplified when a train passes
with its whistle blowing, by the sudden drop in
pitch of the note of the whistle as the train
passes and instead of moving towards us is moving
away.

The radiations that compose the light received
from the distant universes are of longer wave-length
than corresponding radiations from terrestrial
sources. This is the observational fact which is
interpreted as due to motions of recession of the dis-
tant universes. But is this necessarily the only in-
terpretation ? It has been suggested, for instance,
that the light from these remote systems is in some
way modified in the course of its long journey
through space. Whether the observed changes in
wave-lengths of the radiations from the remote
systems are due to their motions or to a slow pro-
gressive change in wave-length as the light travels
through space is a question that can ultimately be
decided by observation. It is hoped that the great
two-hundred-inch telescope, now in course of con-
struction, will make a decision between the two
alternatives possible; the great light-gathering

246 LIFE ON OTHER WORLDS

power of this giant telescope, enabling it to explore
space to much greater distances than any existing
telescope, will be invaluable for this purpose.

If these observations prove that the conception of
the expansion of the universe is not correct, we shall
no longer have any reason to suppose that the
average distances of the stars from one another were
ever much different from what they now are. If,
on the other hand, the observations prove that the
universe as a whole is now in a state of expansion,
what can we legitimately infer about its past his-
tory ? To suppose that there has been a uniform
expansion from an initial highly condensed condi-
tion is to make an assumption which, though it may
possibly be correct, is without any justification from
observation. The course of events in a changing
universe can be discussed by mathematics, and it
appears that there are various possibilities: a
uniform expansion is one; pulsation, with alternate
periods of expansion and contraction, is another.
We have no means of deciding which course the
universe has actually followed. It may be true, or
it may not, that the average distance apart of the
stars was at one time much less than it is at present;
planetary systems may be relatively few in number
or, on the other hand, they may be far more
numerous than was believed until recently.

Whichever view we incline to take, however, we
are certainly not justified in supposing that the solar
system is unique. It has somehow come into exist-
ence, and it is not logical to suppose that other
systems could not come into existence in a similar
way. The probability might be extremely small,

BEYOND THE SOLAR SYSTEM 247

and yet the number of planetary systems in the
whole universe could be considerable, because the
number of stars in each of the separate stellar
universes and the number of these universes are
both very great. It has been estimated that there
are about 100 million universes in the region of
space that can be probed by the one-hundred-inch
telescope; if the number of stars with a family of
planets did not average more than one per universe,
the total number of planetary systems would still be
considerable. It seems to me that we cannot avoid
the conclusion that the total number of planetary
systems must be very large.

But the existence of other planetary systems,
though a necessary condition for life to exist else-
where in the universe, is not a sufficient condition.
In any planetary system everything seems to be
weighted against the possibility of the existence of
life; a somewhat precise adjustment of conditions is
needed in order that life may be possible. If the
planet is very near its parent Sun, it will be too hot
for life to exist; if it is very far away, it will be too
cold. If it is very much smaller than the Earth, it
will have been unable to retain any atmosphere.
If it is much larger, it will have retained too much
atmosphere; for when the gravitational attraction is
so great that hydrogen cannot escape from the
atmosphere the formation of the poisonous gases,
ammonia and marsh-gas which we found in the
atmosphere of Jupiter and Saturn appears to be
almost inevitable. There seems to be little chance
that life can exist on any world if that world differs
greatly from the Earth in size and weight; it must

248 LIFE ON OTHER WORLDS

be neither very much smaller than the Earth nor
very much larger.

But this is not the only restriction. The stars
differ enormously in candle-power or luminosity.
There are some stars, called giant stars, whose
luminosity is many thousands of times greater than
that of the Sun. There are other stars, called
dwarf stars, whose luminosity is very much smaller
than that of the Sun. Thus, for instance, Canopus
is about eighty thousand times more luminous than
the Sun; the Sun, on the other hand, is about sixteen
thousand times more luminous than the faint com-
panion of the bright star Procyon. If the Sun were
replaced by Canopus, the Earth would become so
intensely hot that all life, both plant life and animal
life, would cease at once. The surface of the Earth
would be seared as though by the blast from a
furnace and the oceans would be rapidly vaporised.
If the Sun were replaced by the companion of
Procyon, the Earth would receive so little warmth
that all the oceans would be frozen and the cold
would be so intense that life would again be out of
the question. Thus not only are there somewhat
narrow limits to the size of a planet if it is to be
the home of life, but also for any given parent
Sun there are somewhat narrow limits of distance
within which the planet must be situated in
order that it may be neither too hot nor too cold
for life.

In the case of the solar system, if the Earth
were as near the Sun as Mercury is, it would be
too hot for any life to be possible on it; if, on
the other hand, it were as far from the Sun as

BEYOND THE SOLAR SYSTEM 249

Jupiter, it would be much too cold for life to be
possible.

A somewhat precise adjustment of two factors,
the size of the planet and its distance from its parent
Sun, thus seems to be essential if life is to be possible
on the planet. It is not enough to have either
factor satisfied without the other. There may be
many planetary systems entirely devoid of life,
either because the planets are too large or because
they are too small or because they are too near the
parent Sun or are too remote from it. It is not
possible to make any estimate of the proportion of
planets that will fall within the appropriate limits
of size and of distance from the parent Sun, though
the proportion is likely to be small.

To sum up the argument: the conditions needed
for birth to be given to a planetary system may be
so exceptional that amongst the vast number of stars
in any one stellar universe we may expect to find
only a very limited number that have a family of
planets; and amongst these families of planets there
cannot be more than a small proportion where the
conditions are suitable for life to exist. Life else-
where in the universe is therefore the exception and
not the rule. If we could travel through the
universe and survey each star in turn, we should not
find life here, there and everywhere. Occasionally
in our wanderings we should find a star with a
family of planets; few of these could be the home of
life, but some there would be which would comply
with our requirements. If the proportion of
planets on which life can exist is not more than one
in a thousand, or even one in a million, the total

25O LIFE ON OTHER WORLDS

number of worlds that are suitable for life would
yet be considerable, so vast is the scale on which
the universe is constructed.

It may still be objected that even where condi-
tions are suitable for life to exist there may be no
life. We cannot hope ever to have any direct
information about these remote worlds; we can
only be guided by what we have learnt from the
study of worlds near at hand. The evidence that
there is vegetation on Mars is almost conclusive,
and affords very strong presumptive evidence that
life will appear when conditions are suitable for it.

It is idle to try to guess what forms life might
take in other worlds. The human mind cannot
refrain from toying with the idea that somewhere
in the universe there may be intelligent beings who
are the equals of Man, or perhaps his superiors;
beings, we may hope, who have managed their
affairs better than Man has managed his. Neither
the investigations of the astronomer nor the investi-
gations of the biologist can help us in this matter.
It must remain for ever a sealed book. But it is
unlikely that evolution has followed a parallel
course on any two worlds. Small differences in
conditions, in climate, in temperature, in atmo-
sphere and in topography, may prove of funda-
mental importance. As evolution proceeds, it may
well be that here and there it branches off into this
or that direction, when some slight change of
conditions perhaps trivial in itself might have
resulted in a branching off into some different
direction, changing entirely the whole subsequent
development.

BEYOND THE SOLAR SYSTEM 25!

There is one further point to mention, the ques-
tion of the time scale. If we could move not
merely through space but backwards and forwards
in time, we should probably find life appearing,
passing through its sequence of evolution, reaching
its zenith of attainment and then passing away on
one world after another. Our study of the Sun's
family of planets suggested that Mars is a world
long past its prime and that Venus is a world that is
perhaps destined to be the home of life in the future.
The zenith of life will not occur at the same time on
all the worlds that are capable of supporting life: it
may be that life in its most highly developed stages
will not persist on any particular world for more
than a small fraction of its life-history.

So we come to the end of our quest. We have
seen that throughout the whole universe there is an
essential uniformity in the structure of matter.
The same few elements occur everywhere, to the
remotest parts of the universe that we have been
able to study. They are the bricks from which all
matter from its simplest to its most complex forms
are built up. We have seen also that matter
obeys the same laws throughout the universe. The
chemistry of the carbon atom is of fundamental
importance in building up the great variety of the
extremely complex molecules that form the basis of
living matter. The carbon atom must play the
same key part wherever in the universe life may be
found. This leads us naturally to the conclusion
that living matter is possible only under somewhat
specialised and restricted conditions. These re-
strictions serve as a guide in estimating the likeli-

252 LIFE ON OTHER WORLDS

hood that life may be found to occur on this world
or on that world.

We have assumed that if the required conditions
for life to be possible are obtained life will auto-
matically make its appearance. This assumption
may be criticised as unjustifiable in the absence of
any knowledge as to how life originated on the
Earth. Yet it appears more plausible than any
other assumption that we can make and receives
some confirmation from the fact that we have direct
evidence of life though admittedly only of plant
life on Mars, which is the only planet where we
might expect to find any evidence of life at the
present time.

But life is not widespread in the universe. The
blazing suns, which we call stars, are far too hot to
permit of the existence of any but the simplest
chemical compounds. It is only on the cooler
planetary bodies that we can hope to find life. Life
is not possible, however, on the large majority of the
planets: some have no atmospheres, others have
atmospheres that are poisonous; some are too hot,
others are too cold. And not more than a small
proportion of the stars are likely to have any planets
at all. With the usual prodigality of Nature, the
stars are scattered far and wide, but only the
favoured few have planets that are capable of sup-
porting life. The radiations from millions upon
millions of stars are being sent out, and for thousands
of millions of years have been sent out, into space.
An almost insignificant fraction of the radiations
from a star here and a star there is utilised in making
life possible on an attendant planet; the great bulk

BEYOND THE SOLAR SYSTEM 253

of the radiation is merely destined to travel on
endlessly through space.

Yet though these restrictions severely winnow
down the possible abodes of life in the universe, we
cannot resist the conclusion that life, though rare,
is scattered throughout the universe. It may be
compared to a rare plant, whose distribution is
widespread, but of which never more than a single
specimen is found at a time. If life is the supreme
purpose of creation, it may be a matter for some
surprise that its occurrence is so restricted. It
might have been expected that every star would
minister to life; it has, in fact, often been asserted
that this is so. But astronomy gives no support to
this view. The task of the astronomer is to learn
what he can about the universe as he finds it. To
endeavour to understand the purpose behind it and
to explain why the universe is built as it is, rather
than on some different pattern which might have
accorded better with our expectations, is a more
difficult task; for this the astronomer is no better
qualified than anybody else.

INDEX

Absolute zero of temperature, 54

Absorption of light, by Earth's at-
mosphere, 64, 66; by planet's at-
mosphere, 68

Absorption spectrum, 63

Adams, 166

Adel, 141, 166, 167

Age of Earth, 238, 241 ; of stars,
235, 241

Alcohol, 46

Amino acids, 37

Amino group, 36

Ammonia, 28, 49; in atmospheres of
Jupiter and Saturn, 139, 148

Analysis of light, 23

Angular momentum, 215, 218, 229,
233; conservation of, 215

Annihilation of matter, 238

Antoniadi, 162, 191

Argon, discovery of, 82; in Earth's
atmosphere, 93

Arrhenius, 206

Atmosphere of Earth, 45, 78; of
planets, 53; composition of, 79;
weight of, 84; addition of helium
to, 85; loss of hydrogen and
helium from, 88; primitive, 88;
of Mercury, 117; of major planets,
1 35 ; of major planets, constitution
of, 142; of major planets, evolu-
tion of, 143; of Venus, 154; of
Mars, 202

Atmosphere, rate of loss of, 60

Atom building, 238, 240

Atoms, 22; numbers of, 22; vibra-
tions of, 23; nature of, 25; valency
of, 28; metastable, 87

Aurora borealis, 84, 86; non-polar,
86

Barnard, 191, 193
Beginning of time, 242
Black body, 71
Bolometer, 75, in
Boron oxide, 40

British Astronomical Association,
125, 19*

Calcium, 29

Callisto, 104, 120

Canals of Mars, 177, 179, 190;
doubling of, 180, 185, 194;
nature of, 187, 195; appearance
of, 192

Candle power of stars, 6

Canopus, 248

Carbohydrates, 31, 38, 43

Carbon, 27

Carbon dioxide, 45; in atmosphere
of Earth, 45, 79, 95; absorbed by
vegetation, 99 ; in primitive
atmospheres of major planets,
146; in atmosphere of Venus,
1 66

Carbonic acid, 43

Carbon monoxide, 49

Carbon tetrachloride, 31

Carboxyl group, 36

Cassini, 175

Caustic soda, 29

Chains of carbon atoms, 35

Chamberlin, 221

Chemical energy, 44

Chlorine, 28, 49

Chloroform, 31

Chlorophyll, 44

Clouds on Venus, 156, 158, 165; on
Mars, 198

Collisions of molecules, 56 ; of stars,
235> 243

Colloidal state, 39

Colour of Venus, 158; of Jupiter,
128, 150; of Uranus and Neptune,
132, 139; of Mars, 183, 191

202

Composition of Sun, 89-94; f

Earth, 89-94
Copernicus, 117
Craters, lunar, 107
Cyanogen, 40

Depletion of oxygen from Earth's

atmosphere, 98
Di-chlor-methane, 31

255

256 INDEX

Distance of Moon, 105; of stars, 4;
of spiral nebulae, 14, 18; of Mer-
cury, 116; of Jupiter, 122; of
Venus, 1 52 ; of Mars, 1 72

Distribution of stellar universes, 19

Dunham, 140, 166

Earth, age of, 238; velocity of escape
from, 78; atmosphere of, 79;
mean density of, 139

Eclipse of Moon, 112

Electron, 24, 238

Energy from Sun, 42; from oxida-
tion, 45; from fermentation, 47;
from annihilation of matter, 239;
from atom building, 240

Equipartition of energy, 55

Equivalent atmosphere, 84

Europa, 104, 120

Expanding universe, 241

Fats, 31, 35
Fatty acids, 35
Fermentation, 47
Formic acid, 44
Fraunhofer lines, 63

Galileo, 154, 175

Ganymede, 104, 120

Gas, 53

Giant stars, 7

Glycerine, 35

Great Meteor Grater, 106

Hay, W., 130

Helium, 23, 82, 137, 240; in at-
mosphere of Earth, 79; in at-
mosphere of major planets, 1 38

Helium atoms, 25

Helmholtz, 50

Herschel, i, 2, 3, 4, 7, 13, 43, 130,
176

Hesperus, 153

Hooke, 127

Huggins, 138

Hydrogen, in Sun and Earth, 92;
solid, on major planets, 137; in
atmosphere of major planets,
138, H9

Hydrogen atom, 25

Hydroxyl group, 34

Infra-red sensitive plates, 157, 197
lo, 104, 1 20

Island Universes, 13, 14, 241, 244;
distances of, 14

Jeans, 12, 60, 223, 22& 235

Jeffreys, 135, 227

Jupiter, 104, 122; satellites of, 104;
distance, 122; telescopic appear-
ance, 122; belts of, 123, 150;
rotation, 123; evidence of at-
mosphere, 124; currents in at-
mosphere of, 125; Red Spot on,
127, 150; South Tropical Dis-
turbance on, 128, 150; colours,
128; temperature, 129, 149;
mean density, 134; ice coating,
135; atmosphere, 135; spectrum,
138; constitution of atmosphere,
142

Kant, 215, 216

Keeler, 131

Kelvin, 238

Kinetic theory of gases, 54, 60

Krypton, 82

Laplace, 216, 217, 219, 220

Life, conditions required for, 40-49;
origin of, 49; not possible on
Moon, 113; not possible on
Mercury, 120; not possible on
major planets, 151; improbabi-
lity of, on Venus, 170; probability
of, on Mars, 207 ; possibility of, in
other planetary systems, 247;
infrequency of, 249

Light of night-sky, 86

Light-year, 5

Living matter, 31-40

Lowell, 181-189, 191* *93> *94> 1 95>

205
Lyttleton, 232

Mars, 103; velocity of escape from,
103, 173; satellites, 104; distance
1 72 ; oppositions of, 1 72 ; size and
weight, 173; phases, 175; tele-
scopic ^ appearance, 175; rotation,
175; polar caps, 176, 183, 196,
201; surface markings, 176; seas,
177, 182; canals, 177, 179, 187,
190, 195; colour, and changes of,
183, 191 ; vegetation on, 183, 186,
206; water- vapour on, 196; evi-
dence of atmosphere, 197; extent
of atmosphere, 198; clouds, 198;
constitution of atmosphere, 202;
temperature, 203; possibility of
life on, 207

Marsh-gas, see Methane

Maxwell, 131, 217

Mercury, 11, 1 03 ; velocity of escape
from, 103; temperature, 103, 117;
distance, 1 1 6 ; same face of,
turned to Sun, 1 1 6 ; atmosphere,
117, 1 1 8; surface markings, 1 18;
transits of, 119; devoid of life,
120

Metals, most abundant, in Sun and
Earth, 90

Meteorites, 90

Methane, 28, 30; in atmospheres of
major planets, 139, 146

Methyl chloride, 31

Methyl group, 31

Milky Way, 2, 3, 8, 9; rotation of,
i o ; mass of, 1 1 ; number of stars
in, ii

Molecules, 27; velocities of, 54, 56

Moon, 102; velocity of escape from,
102; temperature, 103, in; dis-
tance, 105; surface features, 107;
devoid of water, 109; devoid of
atmosphere, no; phases, in;
devoid of life, 112; gravitation
of, 115; mean density, 134

Moulton, 221

INDEX 257

Nebulae, 3, 8; luminous, 8; dark, 9;
spiral, 13

Neon, 82, 93

Neptune, n, 132; satellite, 104;
colour, 132; rotation, 132; varia-
tions of brightness, 1 33 ; tempera-
ture, 133; mean density, 134; ice
coating, 135; atmosphere, 135;
spectrum, 138; constitution of
atmosphere, 142

Neutron, 24, 122

Nitrogen, 28; in Sun and Earth, 92;
in primitive atmospheres of major
planets, 143; in atmosphere of
Venus, 1 68

Nucleus of atom, 24

Number of stars, 1 1 ; of universes, 18

Occultations, no

Oppositions of Mars, 1 72

Origin of life, 49

Origin of solar system : Kant's hy-
pothesis, 215; Laplace's hypo-
thesis, 216; collisional theories,
219; plane tesimal theory, 221;
Jeans' theory, 223; Jeffreys 1
theory, 227; Lyttleton's theory,
232

Oxidation, 38, 45

Oxygen, 28; needed for life, 46;
absorptions produced by, 66;
in Earth's atmosphere,, 79, 98;
metastable atoms of, 87; in Sun
and Earth, 94; produced by
vegetation, 100; on Mars, 202,
207

Ozone, 64

Pasteurisation, 41

Peek, 151

Phillips, 125, 126

Phosphorus, 153

Photocatylist, 44

Pickering, 182, 195

Planetary systems, frequency of

occurrence, 234, 247
Planetesimal theory, 221

INDEX

Pluto, 214, 218

Polar caps on Mars, 176, 183,

196; nature of, 200; thickness of,

201

Positron, 24
Pressure of gas, 53
Primitive atmosphere of Earth, 88
Procyon, 248
Proteins, 31, 36
Proton, 24, 238
Pulsating stars, 5, 10, 14
Pulsating universe, 246
Pythagoras, 153

Ramsay, 82

Rate of loss of atmosphere, 60

Rayleigh, 82, 86

Rays, lunar, 109

Recession of external universes, 1 5,
241, 245

Red Spot on Jupiter, 127, 150

Reduction, 38

Rings, Saturn's, 131, 217; rotation
of, 131

Rotation of Milky Way system, 10;
of spiral nebulae, 1 3 ; of Mercury,
116; of Jupiter, 123, 126; of
Saturn, 130; of Uranus, 132; of
Neptune, 132; of Venus, 159,
162

Russell, 232

Salt, 28

Satellites, 104

Saturn, 104, 122; satellites, 104;
telescopic appearance, 129; belts
on, 129, 150; spots on, 130; rota-
tion of, 130; temperature of, 131,
149; rings, 131, 217; mean den-
sity, 134; ice coating, 135, at-
mosphere, 135; spectrum, 138;
constitution of atmosphere, 142

Schiaparelli, 118, 176-181

Sedimentary deposits, 98

Shooting stars, 84, 114

Silicon, 27

Silicon fluoride, 40

Slipher, 139, 140, 141, 166, 167, 202

Sodium, 28, 150

Solar system, regularities in, 213;
Kant's hypothesis, 215; Laplace's
hypothesis, 216; collisional the-
ories, 219; planetesimal theory,
221 ; Jeans' theory, 223; Jeffreys'
theory, 227; Lyttle ton's theory,
232

South Tropical Disturbance, 128

Spectroscope, 62

Spectrum, 24, 62

Spiral nebulas, 13; recession of, 15,
241, 245; distances of, 18;
number of, 19

Stars, measurement of distances of,
4; pulsating, 5, 10; candle-power,
6; giant, 7 ; number in Milky Way
system, 1 1 ; distance apart, 1 2 ;
temperatures, 43; age of, 235;
internal temperatures, 249

Stefan, 71

Stellar energy, source of, 238

Sugar, 34, 46

Sun, temperature of, 40; rotation of,
125

Sunlight, energy from, 38

Sunspots, temperature of, 40

Swedenborg, 214

Temperature, absolute zero of, 54;
of Sun, 40; effect on living matter,
41 ; of stars, 43; effect on molecu-
lar velocities, 54; of Moon, 1 1 1 ;
of Mercury, 117; of Jupiter, 1 29 ;
of Uranus, 133; of Neptune, 133;
of Venus, 163; of Mars, 203

Temperature, internal, of Sun, 239

Temperatures of planets, 70; how
estimated, 71 ; how measured, 75;
comparison between estimated
and measured values, 76

Thermocouple, 75, in

Tidal friction, 106, 159

Time scale, 251

Titan, 104

Titanium oxide, 40

Transits of Mercury, 119, 156; of

Venus, 1 1 9, 155
Travers, 82
Tri-chlor-methane, 31

Ultra-violet light, 64
Universe, HerscheFs picture of, 2,
7; position of Sun in, 8; island,

*3

Uranus, 122, 132; discovery of, i;
satellites, 104; colour, 132; belts,
132; rotation, 132; temperature,
133; mean density, 134; ice
coating, 135; atmosphere, 135;
spectrum, 138; constitution of
atmosphere, 142

Valency, 28

Vegetation, on the Earth, 99;

probable absence on Venus, 169;

on Mars, 183, 186, 206, 250
Velocity of escape, 57-61, 102, 154
Velocities of molecules, 54

INDEX 259

Venus, distance, 152; transits, 119,
155; phases, 153; size, 154; velo-
city of escape from, 154; atmo-
sphere, 1 54 ; telescopic appearance,
156; markings on, 156; colour,
158; clouds on, 158; rotation,
I 59i temperature, 163, 170;
oxygen and water-vapour not de-
tected in atmosphere of, 164;
carbon-dioxide in atmosphere of,
1 66

Water, needed for life, 48

Waterfield, 191

Water-vapour, absorptions pro-
duced by, 66; in Earth's atmo-
sphere, 83; on Mars, 196, 202

Wave-length of light, 64

Wildt, 135, 139, 152

Williams, 124, 126

Wright, T., 215

Wright, W. H., 197

Xenon, 79, 83